Russia will face “further consequences” if it does not stop its tanks crossing the border with Ukraine, David Cameron has warned.
"It is simply not enough to engage in talks in Minsk, while Russian tanks continue to roll over the border into Ukraine. Such activity must cease immediately," Mr Cameron said in a statement.
"We urge Russia to pursue a different path and to find a political solution to this crisis. If Russia does not, then she should be in no doubt that there will be further consequences."
His remarks come after a Ukrainian official claimed two columns of Russian tanks and military vehicles fired missiles from Russia at a border post in Ukraine on Thursday and then rolled into the country as its border guards fled.
Earlier, the Ukrainian President cancelled a working trip to Turkey, saying Russian military troops had "actually been brought into the country".
Petro Poroshenko called an urgent meeting of the Ukrainian security and defence council to decide the country's next steps, which is due to begin at 4pm.
"I made the decision to cancel a working visit to the Republic of Turkey in connection with the rapidly deteriorating situation in Donetsk region, in particular in Amvrosiyivka and Starobesheve, as Russian troops were actually brought into Ukraine" he said.
Mr Poroshenko's statement came as the pro-Russian separatist leader Alexander Zakharchenko claimed former and serving Russian soldiers on leave from their posts are fighting Ukrainian troops alongside the rebels.
The rebel leader told Russian state television station: "Among us are fighting serving soldiers, who would rather take their vacation not on a beach but with us, among brothers, who are fighting for their freedom."
In pictures: The Ukraine crisis
In pictures: The Ukraine crisis
1/39 Ukraine crisis
Ukrainian soldiers from the 'Azov' battalion guard their base in Mariupol, Donetsk
2/39 Ukraine crisis
Children were transferred from orphanages in Donetsk and Makeyevka to escape the fighting between the Ukrainian army and pro-Russian separatists
3/39 Ukraine crisis
Ukrainian servicemen captured by pro-Russian separatists sit on the ground as they are assigned to clean a street in Snizhne in the Donetsk region
4/39 Ukraine crisis
A view inside the Youth Culture Centre destroyed by pro-Russian separatists in Donetsk
5/39 Ukraine crisis
A woman holds a portrait of her dead son as she speaks during a rally in front of Ukrainian President Petro Poroshenko's office in Kiev
6/39 Ukraine crisis
A boy ascends the stairs of a bomb shelter after the shelling in the Petrovskiy district in Donetsk
7/39 Ukraine crisis
A medic looks at thirty coffins prepared for the funerals of pro-Russian rebels killed during heavy fighting at Donetsk airport, outside a Donetsk morgue
8/39 Ukraine crisis
Relatives mourn near the coffin of Mark Zverev, a taxi driver shot dead during clashes at the Donetsk airport between Ukrainian troops and the pro-Russian rebels, during his funeral in the village of Grabari on the outskirts of Donetsk
9/39 Ukraine crisis
Medical workers carry a wounded pro-Russian militant on a stretcher after armed clashes occured between pro-Russian gunmen and Ukrainian troops in Slavyansk
10/39 Ukraine crisis
A member of a newly-formed pro-Russian armed group called the Russian Orthodox Army mans a barricade near Donetsk airport
11/39 Ukraine crisis
Black smoke rises from a shot down Ukrainian Army helicopter outside Slovyansk
12/39 Ukraine crisis
A woman is overcome with emotion as she looks at blood stains and damage around a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
13/39 Ukraine crisis
A local man looks at damage near a wrecked truck of supporters of the self-proclaimed 'Donetsk People's Republic' on road leading to the Donetsk International Airport
14/39 Ukraine crisis
A bloodstained icon of Jesus is seen among blood soaked shattered glass atop a wrecked truck near the Donetsk airport
15/39 Ukraine crisis
A family member attends a funeral for Olga Prokhorenko (60) who was killed by shrapnel after Ukrainian government forces shelled their location, during the funeral in Slovyansk
16/39 Ukraine crisis
A woman reacts after seeing the lifeless body of a man killed by shrapnel following a shelling from Ukrainian government forces in Slovyansk
17/39 Ukraine crisis
Pro-Russian militants take position on the roof of the international airport of the eastern Ukrainian city of Donetsk
18/39 Ukraine crisis
A woman embraces a pro-Russian separatist from the "East" battalion during a rally in the eastern city of Donetsk
19/39 Ukraine crisis
A Ukrainian helicopter Mi-24 gunship fires its cannons against rebels at the main terminal building of Donetsk international airport
20/39 Ukraine crisis
A pro-Russian gunman changes his position near the airport, outside Donetsk
21/39 Ukraine crisis
A pro-Russian gunman aims his weapon near the airport, outside Donetsk. Ukraine's military launched air strikes against separatists who had taken over the airport in the eastern capital of Donetsk in what appeared to be the most visible operation of the Ukrainian troops since they started a crackdown on insurgents
22/39 Ukraine crisis
Pro-Russians ride on a truck in Donetsk. A convoy of an armored personnel carrier and seven trucks carrying several hundred heavily armed men drove through central Donetsk and gunmen got out of the trucks, stood to attention and gave shots in the air in jubilation as a crowd of several thousand supporters cheered them and chanted: "Heroes!"
23/39 Ukraine crisis
An elderly woman leans against the chest of a pro-Russian gunman in Lenin square in Donetsk
24/39 Ukraine crisis
Pro-Russian militants guard a psychiatric hospital after shell explosions during the fighting between pro-Russian militants and the Ukrainian army, in Semyonovka village, outside Slavyansk
25/39 Ukraine crisis
Ukrainian troops stand guard at a checkpoint on the road near the eastern city of Izum, Donetsk
26/39 Ukraine crisis
Members of the Democratic Alliance party take part in a performance in front of the French embassy in Kiev. Protesters set up a pool with the blood of animals and models of warships in a protest against French plans for the sale of two Mistral helicopter carriers to the Russian Navy
27/39 Ukraine crisis
A man carries a portrait of pro-Russian activist Vadim Hudich, who was killed in a shooting incident at the head of a funeral procession in the eastern Ukranian town of Krasnoarmeisk
28/39 Ukraine crisis
A voter is seen inside a voting booth at a polling station during the referendum on the status of Donetsk region in the eastern Ukrainian city of Donetsk
29/39 Ukraine crisis
Members of a local election commission sort ballots as they start counting votes of referendum on the status of Luhansk region in Luhansk
30/39 Ukraine crisis
Members of a local election commission empty a ballot box as they start counting votes of the referendum on the status of Donetsk region in Donetsk
31/39 Ukraine crisis
Members of a local election commission empty ballot boxes as they start counting votes after a referendum, at a polling station in Lugansk
32/39 Ukraine crisis
Local residents watch as others give first aid to a man who was shot in the leg by Ukranian militia in the village of Krasnoarmisk. Eyewitness said that Ukranian militia tried to stop the referendum voters briefly taking the City Hall of Krasnoarmisk, where unarmed pro-Russian supporters were gathering. Reportedly the Ukranian militia came out out the building and started shooting at the people, killing at least one man and leaving at least two others injured
33/39 Ukraine crisis
Members of a local election commission count votes after a referendum organized by the so-called Donetsk People's Republic members at a polling station in Donetsk
34/39 Ukraine crisis
Voters visit a polling station to take part in the referendum on the status of Donetsk
35/39 Ukraine crisis
An Armoured Personnel Carrier (APC) with a Russian flag drives through the center of Slaviansk during the day of referendum organized by the so-called Donetsk People's Republic members in Slavyansk
36/39 Ukraine crisis
A statue of Lenin is placed in front of a pro-Russian barricade on the outskirts of the eastern Ukrainian town of Slavyansk
37/39 Ukraine crisis
A feamle veteran (C) sings during 'Victory Day' celebrations in Donetsk
38/39 Ukraine crisis
People stand near the burning Mariupol police station
39/39 Ukraine crisis
Ukrainian soldiers stand guard beside an armoured personnel carrier at a checkpoint in Mariupol. Ukrainian forces seized the rebel-held city hall in the eastern port city of Mariupol, driving out pro-Russian activists, then withdrew, making no attempt to hold onto the building
He estimated between 3,000 and 4,000 Russians have joined fighting, but insisted they did so voluntarily and not on Moscow's orders.
A senior Nato officer in Brussels said an estimated 1,000 Russian troops are operating inside Ukraine and another 20,000 Russian troops are stationed close by.
Zakharchenko spoke as pro-Russian separatists seized the south-eastern resort town of Novoazovsk on the Azov Sea, threatening the key port of Mariupol.
Video: Russian troops might be in Ukraine
Novoazovsk lies along the road linking Russia to Mariupol and on to Crimea, the Black Sea peninsula annexed by Russia in March. The town escaped most of fighting since the conflict began but has been under heavy shelling for three days.
The country's security and defence council said "a counter-offensive by Russian troops and separatist units is continuing in south-east Ukraine."
Ukraine has accused Russian forces of crossing the border into Ukraine and supporting rebels, an accusation Moscow has consistently denied.
On Wednesday, Ukraine's Foreign Ministry began tweeting the hashtags #UkraineUnderAttack and #RussiaInvadedUkraine, while urging others to re-tweet them.
The US government has also accused Russia of orchestrating a new military campaign in Ukraine that is helping rebel forces expand their fight and sending in tanks, rocket launchers and armoured vehicles.
Earlier, the the German Chancellor Angela Merkel phoned the Russian President Vladimir Putin to demand an explanation on claims Russian troops have launched an incursion into Ukrainian territory.
French President Francois Hollande warned it would be "intolerable and unacceptable" if Russian troops are shown to have entered Ukrainian territory.
"Europe will maintain (sanctions), even increase them if the escalation increases. I don't want it because it is neither in Russia's nor our interest," Mr Hollande told an annual conference of French diplomats in Paris on Thursday.
Additional reporting by agenciesReuse content