Artillery pounded the rebel bastion of Donetsk in southeastern Ukraine on Sunday as the West warned Russia that any attempt to send "humanitarian" troops into the conflict-torn region would be "unacceptable".

Ukraine is ready to accept an aid mission to the eastern rebel stronghold of Lugansk, President Petro Poroshenko said, but only if it is an unarmed international team entering the country through Kiev-controlled borders.

In a phone conversation with German Chancellor Angela Merkel, Poroshenko said he was already in discussions with Red Cross chief Peter Maurer over such a possible mission.

The West warned Russia on Saturday that any attempt to enter Ukraine on "humanitarian" grounds would be considered an "illegal" invasion after Kiev claimed Russian troops had tried to cross the border in the guise of aid workers.

Moscow denied the claim, saying "Russian troops made no attempt to penetrate" Ukraine, where pro-Russian rebels in the east admitted that their stronghold Donetsk had been surrounded by Kiev's troops.

Any "humanitarian" mission by Russia into Ukraine would be "unjustified and illegal", U.S. President Barack Obama and British Prime Minister David Cameron agreed during a telephone call Saturday, Downing Street said.

"On Ukraine, both expressed grave concern about reports that Russian military vehicles have crossed the border into Ukraine and that Russian armed forces are exercising for a 'humanitarian intervention,'" a statement from Cameron's office said.

Dutch forensic experts have identified a total of 65 victims of downed flight MH17, the government said on Saturday, as the last of the investigators returned from eastern Ukraine.

Forty-two more victims were identified from remains taken from the crash site of the Malaysia Airlines jet, where an operation to recover body parts has been halted because of rising clashes between the Ukrainian government and pro-Russian separatists.

Russia denied on Saturday that it had attempted to send troops across the border under the guise of a humanitarian mission, and pledged it would not make such a unilateral intervention.

"Russian troops made no attempt to penetrate" into Ukraine, Russian President Vladimir Putin’s spokesman Dmitry Peskov was quoted as saying by Interfax news agency.

Fears mounted Saturday in Ukraine of a possible Russian invasion in the guise of a "humanitarian" mission to the conflict-torn east where Kiev said another 13 of its troops died in fighting with pro-Moscow rebels.

The United States has warned that any unilateral intervention by Russia, including in the form of a humanitarian mission, would be considered an invasion.

The Red Cross on Saturday denied involvement in a Russian "humanitarian convoy" that Ukraine claimed to have stopped from crossing the border over fears it was an attempt by Moscow to invade the ex-Soviet state.

The charity said while it had received an offer from Moscow to organize aid convoys, and would welcome any effort to ease civilian suffering in the region, no such action had taken place.

Fierce battles on Ukraine's porous eastern border left 15 government troops dead as fears of a possible Russian invasion swirled on Friday despite NATO urging Moscow to withdraw its troops along the frontier.

International tensions also rose as Western countries slammed a Russian food embargo imposed as revenge for sanctions slapped on Moscow over its backing for insurgents in Ukraine.

Ukrainian police on Friday detained a man after a grenade blast injured two people close to Kiev's Independence Square, the interior ministry said.

The man hurled the explosive device at the director of an exhibition center where some demonstrators from the remains of the protest movement that toppled president Viktor Yanukovych in February are still squatting, it said in a statement said.