At least four Ukrainian servicemen were killed and 12 wounded vernight in the military campaign against pro-Russian separatists in the east of the country, the military and the border guard service said on Friday.

Two soldiers and at least one border guard were killed after their armoured vehicle drove over a landmine near Dovzhansky on Ukraine's border with Russia, they said.

Military spokesman Vladyslav Seleznyov said a soldier was also killed in an incident near the town of Karlovka in the region of Donetsk, the main city where rebel fighters are holding out against the government forces.

He said at least 50 separatist fighters had been killed in the last few days following air strikes against rebel positions near the border with Russia.

There was no immediate comment from the rebels.

The government forces have gained the upper hand in the three-month-old conflict in the Russian-speaking eastern regions in which more than 200 government troops have been killed as well as hundreds of civilians and rebel fighters.

In diplomacy to end the worst Russia-West crisis since the Cold War, French President Francois Hollande and German Chancellor Angela Merkel urged Russian President Vladimir Putin by telephone on Thursday to exert pressure on the separatists to de-escalate the violence.

Ukrainian President Petro Poroshenko told U.S. Vice President Joe Biden that Russia and Ukrainian separatists had refused multiple proposals by Kiev for venues to negotiate a ceasefire, the White House said.

Rebels armed with heavy machine guns took positions late on Thursday around the airport controlled by Ukrainian forces in the industrial hub of Donetsk. Shots were heard overnight.

"Last night for an hour there was shelling against Ukrainian servicemen at the Donetsk airport," Seleznyov wrote on his Facebook page.

Poroshenko has ruled out air strikes and artillery bombardment because of the large civilian population in Donetsk.

Bt the Ukrainian military say they have a plan to deliver a "nasty surprise" to the rebels and take back their main strongholds, Donetsk and Luhansk.