Ukraine on Monday accused Russian army officers of fighting alongside separatists in the east of the country and said Moscow was once more building up its troops on the joint border.
A missile that brought down a Ukrainian transport plane near the border was probably fired from Russia, Ukrainian officials said.
President Petro Poroshenko held an emergency meeting of his security chiefs after a weekend of Ukrainian air strikes on rebel positions near the border with Russia and charges by Moscow that Kiev killed a Russian man with a cross-border shell.
The war of words between Kiev and Moscow and intense fighting, in which Ukrainian forces say they inflicted heavy losses on the rebels, marked a sharp escalation in the three-and-a-half-month conflict in which several hundred Ukrainian servicemen, civilians and rebels have been killed.
“Information has... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces,” Mr Poroshenko said.
Ukraine crisis: Bridges blown up as rebels retreat
Ukraine crisis: Bridges blown up as rebels retreat
People walk under a destroyed railroad bridge over a main road leading into the east Ukraine city of Donetsk
A railway bridge, damaged during the night of 6-7 July, near the village of Novobakhmutivka, 30 km north of Donetsk
A bus drives under the bridge which was blown up recently by Pro-Russian militants in the near of Donetsk
A Ukrainian government soldiers guards wrecked tanks and armored personnel carriers (APCs) left by pro-Russian insurgents in the city of Slavyansk
Mines lay near a destroyed pro-Russian APC near the city of Slovyansk
A sign " Danger mines" is set at the check-point near Slavyansk on the road to Kramatorsk
Smoke rises outside the outskirts of the eastern Ukrainian city of Donetsk. Retreating pro-Russian insurgents dug in on 7 July in Ukraine's sprawling industrial hub of Donetsk after government forces scored a string of morale-boosting victories in the bloody battle for the future of the country. The rebels erected checkpoints along the main roads leading into Donetsk while the center of the riverbank city itself saw several restaurants and shops shut their doors
The facade of a building in the eastern Ukrainian city of Kramatorsk, destroyed after fightings between Ukrainian troops and pro-Russian militants
A woman looks out from her damaged house in the city of Slovyansk
Armed pro-Russian militants have an old Russian-made tank from the local WWII museum transported to near the Donbass-arena stadium in Donetsk. Reportedly the separatists intend to use it in possible fighting with the Ukrainian army. Ukrainian officials announced a total blockade of the two cities, after government forces took the separatist stronghold of Sloviansk and several other cities in the Donetsk region
Mr Poroshenko made similar complaints of Russian incursions on Sunday to the European Union with an eye to pushing the bloc to exert greater pressure and possibly more sanctions on Moscow. Mr Poroshenko told his security chiefs that government forces, which lost 23 men in a rocket attack on an army camp last Friday, were now facing a new Russian missile system and there would have to be a change in tactics. He gave no details.
Accusing Russia of embarking on a course of escalation in Ukraine’s eastern regions, the National and Security Council spokesman Andriy Lysenko told journalists: “In the past 24 hours, deployment of [Russian] units and military equipment across the border from the Sumy and Luhansk border points was noticed. The Russian Federation continues to build up troops on the border.”
Nato said Russia had increased its forces along the border and now had 10,000 to 12,000 troops in the area.
Moscow’s response to the cross-border shelling and the Ukrainian reports of Russian troops being moved up to the border raised again the prospect of Russian intervention, after weeks in which President Vladimir Putin had appeared intent on disengaging, pulling back tens of thousands of troops he had massed at the frontier.
Video: Families flee from Donetsk
The Ukrainian army said it had broken a rebel encirclement of Luhansk airport on Sunday night. A spokesman for the so-called Luhansk People’s Republic said 30 volunteer fighters had been killed in Ukrainian fire on Oleksandrivka, a village to the east of the town, Russia’s Interfax news agency said.
As military action continued on Monday near the rebel-controlled border town of Luhansk, Ukraine’s Defence Minister said a Ukrainian AN-26 transport plane, taking part in the military campaign against the rebels, had been shot down by a rocket which was “probably” fired from Russian territory.
Officials said crew members had been in contact with the army general staff. They were believed to be two in number but there was no word on casualties.
Mr Lysenko said separatists, backed by what he described as Russian “mercenaries”, had fired on Ukrainian border guards in an attempt to give cover as armoured vehicles and equipment were being brought into the country.
And he again rejected Russian charges that Ukraine forces had fired a shell over the border killing a Russian man on Sunday– an incident that Moscow has described as an “aggressive act” which would have “irreversible consequences”.
“The [rebel] fighters systematically fire mortars and shoot into Russian territory, which killed a Russian citizen,” Mr Lysenko told journalists. Russia said it had invited monitors from the OSCE, a European security and rights body, to visit two of its border crossings with Ukraine as a sign of goodwill.