Artillery continues to pound the rebel bastion of Donetsk in southeastern Ukraine as the West warns Russia any attempt to send "humanitarian" troops into the conflict-torn region will be "unacceptable".
In a round of telephone calls late on Saturday, US President Barack Obama, German Chancellor Angela Merkel and British Prime Minister David Cameron agreed that any unilateral move by Russia into Ukrainian territory would be seen as "illegal" and "unacceptable".
Russia on Saturday denied an allegation by Kiev that it had tried to send in a convoy disguised as an aid mission.
Shelling meanwhile continued through the night in Donetsk, a city of one million that rebels now say has been surrounded by Ukrainian forces.
There were no reports of casualties however.
Western leaders reacted strongly after Ukraine said it had scuppered a Russian "humanitarian convoy with 'peacekeepers'" moving towards the border, accompanied by troops and military hardware.
"Any Russian intervention in Ukraine ... without the formal, express consent and authorisation of the government of Ukraine is unacceptable," Obama and Merkel agreed in a phone call late on Saturday.
Such a move "violates international law, and will provoke additional consequences," on top of the many economic sanctions already in place against Moscow, the White House added.
In a separate conversation, Obama and Cameron said a Russian move into Ukraine would be "unjustified and illegal".
The West has warned for days that Moscow, whom it accuses of backing the separatists, could use the looming humanitarian crisis in eastern Ukraine as a pretext to send in troops.
NATO says Russia has about 20,000 troops on the border with its former Soviet neighbour.
A spokesman of Russian President Vladimir Putin denied on Saturday that Moscow tried to enter Ukraine's territory as claimed by Kiev.
Foreign Minister Sergei Lavrov however later telephoned his US counterpart John Kerry, seeking support for an aid mission to southeastern Ukraine.
More than 285,000 people have fled their homes in eastern Ukraine, according to the UN refugee agency UNHCR.
Fighting between government troops and insurgents has also left 1300 people dead and more than 4000 injured in four months of what the Red Cross has already deemed a civil war in the industrial region.
Local authorities in the east are now warning of a looming humanitarian catastrophe, especially in the second largest rebel-held city of Lugansk, where residents have been without power or running water for days, while fuel and food supplies are running short.
Pensions, salaries and social benefits were also not being paid out as many banks in the region are closed, authorities said.
Donetsk meanwhile came under renewed shelling overnight and on Sunday morning, with city authorities reporting a home and a clinic in a district north of the centre had been hit, injuring at least one person.
Journalists on the ground said they heard more than 20 explosions in the early morning and that the assault was continuing.
