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.
"We have difficulty understanding what the Ukrainians are talking about," Dmitry Peskov, spokesman for Russian President Vladimir Putin, told the Interfax news agency.
But US President Barack Obama, Britain`s Prime Minister David Cameron and Germany`s Angela Merkel made it clear they would not brook any attempt by Russia to use humanitarian excuses to sneak troops and military equipment into the conflict-torn east of Ukraine.
"The Prime Minister and President are absolutely clear that such a so-called humanitarian mission would be unjustified and illegal," Downing Street said in a statement following a phone call between Obama and Cameron.
Obama and Merkel also spoke on the phone on Saturday, with both agreeing that any Russian intervention would be "unacceptable", the White House said.
The West has accused Russia of abetting the insurgency by supplying it with weapons, and Kiev said it had scuppered an attempt by Russia to send troops across its porous eastern borders under the guise of aid workers.
"A huge convoy moved towards the Ukrainian border, accompanied by Russian troops and military hardware," Valeriy Chaliy, deputy head of President Petro Poroshenko`s office said late on Friday in a television interview.
Although it denied the allegations, Moscow nonetheless called on Western countries to back its plans for a "humanitarian mission" to east Ukraine.
Russian Foreign Minister Sergei Lavrov phoned his US counterpart John Kerry on Saturday "to underline the need for urgent measures to avert the imminent humanitarian crisis" in the region, according to a statement from the Russian foreign ministry.
Poroshenko said his government would be willing to accept an aid mission to the rebel bastion of Lugansk -- where local authorities said was on the verge of a humanitarian catastrophe, but only under strict conditions.
"We are ready to accept humanitarian aid if the mission is an international one, without any military escort and if it passes through border checkpoints controlled by Ukrainian guards," Poroshenko told Merkel in a phone conversation, according to a statement issued by his office.
He added he was already in discussions with Red Cross chief Peter Maurer over a possible mission.Meanwhile, heavy bombardment continued to rock the rebel stronghold of Donetsk, which has become the key battleground in the four-month-old conflict between the Ukrainian military and pro-Russian separatists.
An AFP journalist heard repeated shelling in the city throughout the morning and again in the afternoon. Local authorities said mortar fire hit neighbourhoods north and southwest of the centre, and one person was killed.
Ukraine reported 13 casualties in the last 24 hours.
The new prime minister of the self-proclaimed Donetsk People`s Republic, Alexander Zakharchenko, admitted on Saturday that the city of one million was surrounded by Ukrainian forces.
The rebel leader called for a truce to stop the city becoming a "new Stalingrad" but warned that if Ukrainian forces do not stand down, "the fight will take place in every street, every house, for every meter of our land."
"We will defend our right to freedom and independence... Victory will be ours!" he said in a statement.
The Red Cross announced it was stepping up its aid activities in Donetsk and the second main insurgent bastion of Lugansk.
Local authorities in Lugansk said on Saturday that the situation was "critical," with no power, running water or phone connections for a week now, while fuel had run out and food supplies were running low. NATO says Russia has 20,000 troops along the Ukrainian border, fuelling fears that Moscow could send them into its former Soviet neighbour.
US ambassador to the UN Samantha Power on Friday slammed Russian proposals to set up humanitarian corridors to east Ukraine.
A "unilateral intervention by Russia in Ukrainian territory, including one under the guise of providing humanitarian aid, would be completely unacceptable and deeply alarming and would be viewed as an invasion of Ukraine," she said.
She drew a parallel with the 2008 crisis in South Ossetia, when Russia justified sending troops into the Georgian territory in response to civilian suffering.
Russia`s Deputy Foreign Minister Sergei Ryabkov said Power`s comments demonstrated the extent of "anti-Russian hysteria" in Washington.
"Our proposal has clearly humanitarian objectives but our initiative is tossed aside and they only talk about how Russia would supposedly try to slip into Ukraine under the guise of humanitarian aid," he told ITAR-TASS news agency.
Tensions between Russia and the West have hit their highest point since the Cold War over the crisis in Ukraine, leading to tit-for-tat sanctions.
So far, the conflict has claimed more than 1,300 lives, according to the United Nations.
Some 285,000 people have also fled their homes in four months of what the Red Cross calls a civil war.