MOSCOW -- Ukrainian troops surrounded pro-Russia separatists in their main stronghold of Donetsk Saturday, prompting the new leader of the militants to appeal for a ceasefire to avert what he said was a looming humanitarian catastrophe.
Alexander Zakharchenko, who on Thursday took over as prime minister of the insurgents' self-proclaimed People's Republic of Donetsk, said civilians had been killed during overnight shelling by Ukrainian government troops and the toll would rise dramatically if the assault was not halted.
"We are ready for a ceasefire to prevent an increasing humanitarian catastrophe in Donetsk," he said, according to Russian news reports. He said water and electricity were scarce in the city, once home to a million people, and food and medicine were also running out after fighting cut off supplies weeks ago.
Luhansk, the other eastern Ukraine city still marginally under the control of pro-Russia gunmen, has been direly short of water, and electrical service and communications have been severely affected for weeks. More than half of its 465,000 residents have fled, mostly across the border to Russia.
There was no immediate response from the government in Kyiv to the insurgents' call for a truce, but Ukrainian security officials said Saturday they had prevented armoured Russian convoys from crossing into the separatist-held territory under the guise of a peacekeeping mission.
Valeriy Chaliy, deputy head of Ukraine's presidential administration, said in Kyiv that Russia had attempted to send forces into the East late Friday night.
"Bearing in mind numerous violations by the Russian Federation of the state border of Ukraine and continued illegal supply of weapons, armoured vehicles and mercenaries from Russia, Ukraine has solid grounds for concern that the convoy may trigger further escalation," the Ukrainian Foreign Ministry said in a statement.
In Moscow, Kremlin spokesman Dmitry Peskov denied Russia was attempting to provide military backing.
"We have difficulty understanding what the Ukrainian side has in mind," he said, Russian news agency RIA Novosti reported.
Peskov said the intensifying humanitarian crisis in eastern Ukraine was causing the "deepest concern" in Russia.
U.S. Secretary of State John Kerry called Russian Foreign Minister Sergei Lavrov Saturday to discuss the deteriorating situation in the separatist-held areas of eastern Ukraine, with their offices providing starkly contrasting accounts of the conversation.
A senior State Department official said Kerry warned Lavrov against any attempts to send Russian personnel or assistance into Ukrainian territory without the consent and co-ordination of the Kyiv government.
Kerry "conveyed Russia should not intervene in Ukraine under the guise of humanitarian convoys or any other pretext of 'peacekeeping,' " the official said.
A statement issued by the Russian Foreign Ministry said Lavrov had impressed on Kerry the urgency of providing aid to the encircled civilian communities to prevent "an impending human catastrophe."
Lavrov also talked with British Foreign Secretary Philip Hammond and informed him about Russia's efforts to get humanitarian assistance to those remaining in the embattled eastern cities.
After British Prime Minister David Cameron and U.S. President Barack Obama conferred, the British leader's office issued a statement rejecting Russia's attempts to come to the separatists' aid.
"The prime minister and president are absolutely clear that such a so-called humanitarian mission would be unjustified and illegal. There are already a number of international aid agencies providing appropriate assistance on the ground in eastern Ukraine, and they urge Russia to desist from such a move," the statement said.
-- Los Angeles Times