Ukrainian military officials say government forces have pulled back from the airport of the eastern city of Luhansk following clashes with pro-Russian separatists.
Military spokesman Andriy Lysenko said the soldiers made an "organized retreat" from the airport on September 1 in the face of an intensifying assault that he blamed on "professional artillery gunmen of the Russian armed forces."
Lysenko said troops had been battling a Russian tank battalion there.
He added that there were “no fewer than four [Russian] battalion-tactical groups” in Ukraine, adding that each one comprised 400 men.
The announcement came shortly before Ukrainian and Russian officials, separatist rebels, and OSCE representatives met in Minsk, the capital of neighboring Belarus, to discuss the Ukraine crisis.
Those talks reportedly lasted four hours but no details were announced.
The sides were expected to meet again on September 5, according to a pro-Russian rebel leader who attended the September 1 meeting.
Russian Foreign Minister Sergei Lavrov had expressed hope participants would agree terms of an immediate, unconditional cease-fire.
Interfax news agency quoted a rebel representative as saying the separatists wanted recognition of “special status” for the regions they control that would allow them to deepen economic integration with Russia and the Russian-led Customs Union.
The rebels have been fighting Ukrainian government forces since April in a conflict the United Nations says has killed more than 2,600 people.
The provincial capitals of Luhansk and Donetsk are in rebel hands.
The separatists appear to have made gains in recent days after suffering setbacks under an intensified government offensive in the eastern regions of Donetsk and Luhansk last month.
The conflict spread to the Sea of Azov on August 31, when Ukrainian authorities said separatists attacked two Ukrainian patrol boats with artillery fire from land, sinking one of them, in an attack that left two sailors missing. Eight others were rescued and are being treated for burns and other injuries.
Ukrainian authorities released a video of one of those boats burning as it was found by naval forces:
Ukraine and the West accuse Russia of sending troops into Ukraine and supporting the separatists, allegations denied by Moscow.
NATO said last week it believed more than 1,000 Russian soldiers had entered Ukraine in what U.S. President Barack Obama called an "ongoing incursion" by Moscow.
Ukrainian President Petro Poroshenko said on September 1 that the events of the past few days show Russia has launched a "direct and open aggression" against Ukraine.
And in a Facebook post, Ukrainian Defense Minister Valeriy Heletey warned that "a great war has arrived at our doorstep -- the likes of which Europe has not seen since World War II,” adding that the conflict could claim tens of thousands of lives.
But Lavrov told students and teachers at Russia's premier international affairs university on September 1 that there would be no "military intervention" in eastern Ukraine.
Lavrov said an immediate truce is needed but that Poroshenko's cease-fire plan amounted to a demand that separatists disarm and allow themselves to be destroyed. He said the separatists had no choice but to try to drive Ukrainian government forces out of positions from which they can hit civilians.
Lavrov also said Russia wants to return to "pragmatic" relations with the West but would not respond to the "language of sanctions," which he said would not change Moscow's policies.
The conflict in eastern Ukraine began after Russia annexed Crimea, a Black Sea peninsula that had been part of Ukraine since the Soviet era, and has brought Russia's relations with the United States and EU to a post-Cold War low.
Western nations have imposed visa and trade sanctions on Russia, which has responded by banning most Western food imports.
Putin indicated on September 1 that he would like to see an end to the sanctions.
"I hope that common sense will prevail and that we will work in a normal, modern fashion. And neither we nor our partners will suffer losses from these mutual jabs," he said.
With reporting by Reuters, AFP, AP, ITAR-TASS, and Interfax