Russia denied allegations Saturday that it tried to send "humanitarian" troops into conflict-torn eastern Ukraine, as rebels in the surrounded stronghold of Donetsk pledged to keep fighting.
Kiev said earlier that it had managed to halt a Russian convoy through diplomatic channels, after the United States warned that any unilateral intervention by Moscow would be "deeply alarming" and considered an invasion.
Rebels meanwhile called for a truce on humanitarian grounds but said they were willing to fight for "every street" of the city, which has become the key battleground in the four-month-old conflict between Ukrainian government forces and eastern 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's forces, which have been battling to wrest control of eastern strongholds from the rebels, who the West believes are backed by Russia, reported 13 casualties in the last 24 hours.
The Red Cross also announced it was stepping up its aid activities in Donetsk and the second main insurgent bastion of Lugansk, where local authorities have warned of a looming humanitarian catastrophe with many residents left without water and power for days.
- 'Unacceptable' intervention -
A top official from Ukrainian President Petro Poroshenko's office said late Friday that Kiev had managed through diplomatic channels to halt a Russian convoy, including troops and military hardware, advancing towards the border under the guise of a humanitarian mission.
"The Ukrainian side has reasonable grounds to believe that the convoy could be used to further escalate tensions," the foreign ministry added on Saturday.
But Moscow denied the allegations.
"We have difficulty understanding what the Ukrainians are talking about. Russian troops made no attempt to penetrate" into Ukraine, President Vladimir Putin's spokesman Dmitry Peskov was quoted as saying by Interfax news agency.
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.
- Critical 'humanitarian' situation -
In Donetsk, the new prime minister of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, admitted Saturday that the city of one million was surrounded by Ukrainian forces.
The city has been at the heart of fighting in recent days as government forces seek to recapture rebel strongholds and cut them off from the Russian border.
Zakharchenko 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.
An AFP reporter heard loud explosions in two different locations around 6:00 am (0300 GMT) and again later in the morning. Apartment blocks and a hostel in the Kievsky district north of the city had shattered windows and there was shelling damage inside flats, AFP witnessed.
A local resident said: "It was the Ukrainian authorities. They have been firing on the Donetsk People's Republic all morning from the direction of the airport."
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 has already called a civil war.
Citing the "deteriorating humanitarian and security environments" in east Ukraine, the Red Cross announced it was stepping up its activities there.
Local authorities in Lugansk said 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.