Russia is said to be boosting its troop presence in eastern Ukraine and attacks from separatist rebels have intensified as they seek to gain more territory from government forces.
Kiev's forces confronted Russian regular troops at an army checkpoint just west of Luhansk after coming under attack there on Tuesday while fighting between government soldiers and the Russian-backed separatists raged on among the ruins of the airport in the city of Donetsk.
In Moscow, Russian Foreign Minister Sergei Lavrov kept up denials that Russia was militarily involved in the conflict in eastern Ukraine in which more than 4,800 people have been killed. He challenged Kiev to "present the proof".
Ukraine says Russian troops engaged in the east remove identifying tags and insignia from their uniforms to hide their affiliation, much as they did in the early days of the seizure of Crimea by pro-Russian forces culminating in its annexation by Russia in March.
"A significant number of Russian soldiers without insignia have been spotted in Luhansk region," military spokesman Andriy Lysenko said. "There is continued mass shelling of positions and a desire by rebels to expand the territory under their control."
One Ukrainian serviceman had been killed and 40 were wounded in fighting.
Separately, Ukrainian Prime Minister Arseny Yatseniuk said his government would be seeking to increase the size of the armed forces by 68,000 personnel up to a total of a quarter of a million. Last week, parliament extended mobilisation and re-introduced conscription.
Ukraine's charges against Russia were certain to ratchet up tension between Kiev and Moscow before a meeting in Berlin of four foreign ministers that should bring Ukrainian Foreign Minister Pavlo Klimkin face to face with Russia's Mr Lavrov.
Kiev wants a return to the peace process based on a plan that emerged from meetings between Ukraine, Russia and separatist leaders under the auspices of the Organization for Security and Cooperation in Europe in Minsk, Belarus, in September.
A ceasefire declared at the Minsk meeting never got off the ground and hundreds of civilians, Ukrainian soldiers and separatists have been killed since.
Kiev has further accused the separatists of seizing 200 square miles of territory in eastern Ukraine beyond separation lines agreed in Minsk.
Mr Lavrov acknowledged pro-Russian separatists in eastern Ukraine held more territory than was assigned to their control under the agreement.
He said Russia had received assurances from the separatists they would retreat to the agreed separation lines.
But he stuck to Russian denials of direct military involvement.
"Before demanding from us that we stop doing something, please present proof that we have done it," he said at a Moscow news conference.
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