Three Ukrainian soldiers have been killed fighting pro-Russian rebels, the government said Thursday, as fighting engulfed a key airport in the east of the country.

The latest clashes rocked a shaky truce as Ukrainian President Petro Poroshenko traveled to Italy for talks on the conflict with European leaders and Russian counterpart Vladimir Putin Friday.

Ukrainian President Petro Poroshenko on Thursday discussed the crisis in his country's east with U.S. President Barack Obama before jetting off for talks with Russia's Vladimir Putin aimed at resolving the conflict.

The two presidents stressed that the battered ceasefire Kiev inked with pro-Moscow rebels in the east "must be strictly maintained and observed by all parties" and discussed the need for Kiev to reestablish control over its border with Russia, the Ukrainian presidency said in a statement.

President Vladimir Putin accused his US counterpart Barack Obama of a hostile approach towards Russia, warning in a Cold War-style tirade that Moscow would not be blackmailed by the West over Ukraine.

Putin fired off his combative comments shortly before he arrived amid tight security to a red carpet welcome in Belgrade, seeking to cement Russia's influence in its loyal European ally.

Russian President Vladimir Putin will meet his Ukrainian counterpart Petro Poroshenko in Milan on Friday, Italian Prime Minister Matteo Renzi said Wednesday.

Renzi said the two men at the center of Europe's worst security crisis in years would meet at 8:00 am (0600 GMT) on the sidelines of a summit of Asian and European leaders (ASEM).

Georgia on Wednesday warned Russia against taking further steps to integrate its separatist region of Abkhazia, saying it would create new security problems in Europe.

President Giorgi Margvelashvili issued an "emergency statement" after the leader of the Kremlin-backed rebel statelet on Monday submitted a draft agreement to its local parliament that would dramatically strengthen ties with Moscow.

German Chancellor Angela Merkel said Wednesday there were plans for her to meet Russian President Vladimir Putin this week and that she hoped for "an open exchange of opinions" on the Ukraine crisis.

Both leaders will attend a two-day Asia-Europe meeting of leaders in Milan, Italy, starting Thursday and "there are plans for talks with the Russian president," Merkel said.

A "rogue" band of pro-Russian rebels was surrounding more than 100 Ukrainian troops in the eastern Lugansk region on Wednesday, the local governor said.

Several injured soldiers have been captured by the fighters, who do not answer to the main separatist leadership in Lugansk, the pro-Kiev governor Gennadiy Moskal said in a statement on his website.

Russian Prime Minister Dmitry Medvedev thinks a reset in relations between Moscow and Washington is "impossible" while punishing Western sanctions remain in force, he said in an interview aired by U.S. TV Wednesday.

Medvedev blasted U.S. and EU measures imposed on Russia over its role in the Ukraine crisis as "absolutely destructive" and "stupid" and said there was no chance of mending ties with the White House while they were in place. 

Seven civilians died and 17 were injured when shells exploded near a funeral procession in a suburb of the eastern Ukrainian city of Mariupol on Tuesday, city hall said.

"Several shells exploded on the territory of the village (of Sartana). Civilians participating in a funeral procession were hurt," it said in a statement, blaming the attack on pro-Russian rebels.

The European Union told Belgrade on Tuesday it should prove its credentials as a future member during an upcoming visit by Russian President Vladimir Putin to Serbia, a historic Moscow ally.

The EU has declared Putin persona non grata for his role in the Ukraine crisis but he can still count on a warm welcome Thursday in Belgrade, which has refused to emulate the tough sanctions imposed on Russia by Brussels.