Ukraine crisis Russia Tanks
Pro-Russian fighters on a T-64 tank in Starobesheve, south east of DonetskFRANCISCO LEONG/AFP/Getty Images

Government troops have withdrawn from a strategic airport in eastern Ukraine, where Kiev said its forces were fighting a Russian tank battalion.

A military spokesman said Ukrainian troops were ordered to pull out from Luhansk airport after night clashes with Russian-backed rebels.

"In the Luhansk direction, Ukrainian forces have received an order and have pulled back from the airport," Ukraine army spokesman Andriy Lysenko said, adding seven Ukrainian soldiers were killed over the past 24 hours.

Lysenko said government forces had come under fire from "professional artillery gunmen of the Russian armed forces".

Earlier, the military said it had been fighting for control of the airport with a column of Russian tanks.

The development came as separatist forces made substantial gains during a counter-offensive in the Luhansk and Donetsk regions.

Moscow continues to deny it directly or indirectly supports the rebels, while Nato has said more than 1,000 Russian soldiers are operating inside Ukrainian territory and another 20,000 have been amassed on the border.

Lysenko also confirmed two Ukrainian coast guard cutters were hit by missiles in the Sea of Azov, off the coast of Mariupol, where rebels recently opened a new front. One ship sank while the other one was heavily damaged, the army spokesman said.

Meanwhile, Ukrainian and Russian diplomats are due to meet for a fresh round of talks in the Belarusian capital of Minsk.

Ahead of negotiation rounds, Russian Foreign Minister Sergey Lavrov said Kiev must push for an immediate and unconditional ceasefire and its troops should retreat from all areas where they could cause harm to civilians.