Fighting has erupted between Ukrainian government forces and Russian-backed separatists in eastern Ukraine, with claims four Ukrainian servicemen have been killed.
Gennady Moskal, governor of Luhansk region on the border with Russia, said on his website that separatists opened fire on government positions with mortar and artillery near the village of Katerinovka.
"According to preliminary data, four Ukrainian servicemen have been killed and two others have been wounded, one of them seriously," Moskal said.
The fatalities could not immediately be confirmed by the military in Kiev.
Though a ceasefire was declared after peace talks in Minsk, Belarus, in February, skirmishes and violations have continued with a daily mounting casualty toll from fighting mostly north-west of the city of Donetsk and near the coastal town of Mariupol on the Sea of Azov.
Ukrainian President Petro Poroshenko said last week that 83 Ukrainian servicemen had been killed since the ceasefire theoretically came into force in February.
Since then, five Ukrainian servicemen were killed over the weekend alone. The separatists have not given any casualty figures.
The news follows the capture of two wounded men who say they are Russian soldiers on active duty in the rebellious east. Ukrainian authorities claim they are definitive proof that Russia is at war in Ukraine, but Moscow hotly denies this.
Representatives of Amnesty International and the Organisation for Security and Cooperation in Europe, which has shepherded the shaky ceasefire, visited the men and found them apparently in reasonable condition.
"I got professional medical aid, surgery. To my relatives, I want to say everything is fine with me, I'm alive and well," said one, who identified himself as Yevgeny Yerofeyev, a captain in the Russian army.
"I'm missing my family and relatives so much," the other, Sergeant Alexander Alexandrov, told reporters who accompanied the international representatives.
In video statements posted by the Ukrainian Security Service, the men said they were taking part in a reconnaissance operation in the Luhansk region at the weekend when they were fired on, wounded and captured. Both say they were members of an army brigade based in the Russian city of Togliatti and had been deployed in Ukraine for more than a month.
Throughout the yearlong fighting in eastern Ukraine in which at least 6,100 people have died, Russia has consistently denied assertions from Kiev and the West that Moscow is providing soldiers and equipment to back up the pro-Russia rebels. The statements by Yerofeyev and Alexandrov directly challenge that denial.
"We're dealing with real soldiers of the armed forces of the Russian Federation," said Ukrainian Security Service spokesman Markiyan Lubkivskiy.
Ukraine crisis: A timeline of the conflict
Ukraine crisis: A timeline of the conflict
1/22 30 November 2013
Public support grows for the “Euromaidan” anti-government protesters in Kiev demonstrating against Yanukovych’s refusal to sign the EU Association Agreement as images of them injured by police crackdown spread.
2/22 20 February 2014
Kiev sees its worst day of violence for almost 70 years as at least 88 people are killed in 48 hours, with uniformed snipers shooting at protesters from rooftops.
3/22 22 February 2014
Yanukovych flees the country after protest leaders and politicians agree to form a new government and hold elections. The imprisoned former Prime Minister, Yulia Tymoshenko, is freed from prison and protesters take control of Presidential administration buildings, including Mr Yanukovych's residence.
Genya Savilov/AFP/Getty Imageses
4/22 27 February 2014
Pro-Russian militias seize government buildings in Crimea and the new Ukrainian government vows to prevent the country breaking up as the Crimean Parliament sets a referendum on secession from Ukraine in May.
Sean Gallup/Getty Images
5/22 16 March 2014
Crimea votes overwhelmingly to secede from Ukraine and join Russia in a ballot condemned by the US and Europe as illegal. Russian troops had moved into the peninsula weeks before after pro-Russian separatists occupied buildings.
6/22 6 April 2014
Pro-Russian rebels seize government buildings in the eastern cities of Donetsk, Luhansk and Kharkiv, calling for a referendum on independence and claiming independent republic. Ukraine authorities regain control of Kharkiv buildings on 8 April after launching an “anti-terror operation” but the rest remain out of their control.
7/22 7 June 2014
Petro Poroshenko is sworn in as Ukraine's president, calling on separatists to lay down their arms and end the fighting and later orders the creation of humanitarian corridors, since violated, to allow civilians to flee war zones.
8/22 27 June 2014
The EU signs an association agreement with Ukraine, along with Georgia and Moldova, eight months after protests over the abandonment of the deal sparked the crisis.
LOUISA GOULIAMAKI/AFP/Getty Images
9/22 17 July 2014
Malaysian Airlines flight MH17 is shot down over eastern Ukraine, killing all 298 people on board. Ukrainian intelligence officials claim it was hit by rebels using a Buk surface-to-air launcher in an apparent accident.
10/22 22 August 2014
A Russian aid convoy of more than 100 lorries enters eastern Ukraine and makes drop in rebel-controlled Luhansk without Government permission, sparking allegations of a “direct violation of international law”.
11/22 29 August 2014
Nato releases satellite images appearing to show Russian soldiers, artillery and armoured vehicles engaged in military operations in eastern Ukraine.
12/22 8 September 2014
Russia warns that it could block flights through its airspace if the EU goes ahead with new sanctions over the ongoing crisis and conflict
13/22 17 September 2014
Despite the cease-fire and a law passed by the Ukrainian parliament on Tuesday granting greater autonomy to rebel-held parts of the east, civilian casualties continued to rise, adding to the estimated 3,000 people killed
14/22 16 November 2014
The fragile ceasefire gives way to an increased wave of military activity as artillery fire continues to rock the eastern Ukraine's pro-Russian rebel bastion of Donetsk
15/22 26 December 2014
A new round of ceasefire talks, scheduled on neutral ground in the Belariusian capital Minsk, are called off
16/22 12 January 2015
Soldiers in Debaltseve were forced to prepare heavy defences around the city; despite a brief respite to the fighting in eastern Ukraine, hostilities in Donetsk resumed at a level not seen since September 2014
17/22 21 January 2015
13 people are killed during shelling of bus in the rebel-held city of Donetsk
18/22 24 January 2015
Ten people were killed after pro-Russian separatists bombarded the east Ukrainian port city of Mariupol
19/22 2 February 2015
There was a dangerous shift in tempo as rebels bolstered troop numbers against government forces
20/22 11 February 2015
European leaders meet in Minsk and agree on a ceasefire in eastern Ukraine beginning on February 14. From left to right: Belarus President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, France's President Francois Hollande and Ukrainian President Petro Poroshenko.
MAXIM MALINOVSKY | AFP | Getty Images
21/22 13 February 2015
Pro-Russian rebels in the city of Gorlivka, in the Donetsk region, fire missiles at Ukrainian forces in Debaltseve. Fighting continued in Debaltseve for a number of days after the Minsk ceasefire began.
ANDREY BORODULIN | AFP | Getty Images
22/22 18 February 2015
Ukrainian soldiers repair the bullet-shattered windshield of their truck as their withdraw from the strategic town of Debaltseve. Following intense shelling from pro-Russian rebels, Ukrainian forces began to leave the town in the early hours of February 18.
Brendan Hoffman | Getty Images
Both the Kremlin and the Russian Defense Ministry reject the assertion that Yerofeyev and Alexandrov are Russian soldiers. A Defense Ministry spokesman said that they were former soldiers; Moscow has said that any Russians fighting in Ukraine are there only as volunteers.
It remains unclear what measures Russia will take for the two hospitalised men. Lubkivskiy said: "We did not get any request from the Kremlin or the Russian Federation about captured soldiers.... For the moment, there is no question about a transfer [of the men to Russia]".
Amnesty International's Ukraine spokesman, Bogdan Ovcharuk, was quoted by the Interfax news agency as saying the two men have asked to see the Russian consul, but no consular visit has been reported.