Authorities reported that at least one person was killed and several others injured on Sunday, as artillery pounded Ukraine's largest rebel-held city throughout the morning.

There were more than 20 loud explosions around lunchtime, news agencies reported, while shelling overnight caused heavy damage to residential buildings, a hospital and several buses. Heavy fire also continued in the city of Luhansk, which has been without electricity and running water for several days.

Ukrainian government forces surrounded Donetsk on Saturday, cutting the city off from Russia, and prompting rebel leader Aleksandr Zakharchenko to propose a ceasefire "to prevent the proliferation of a humanitarian disaster."

But Western leaders have viewed the truce offer with some apprehension, amid concerns it may be an attempt to push the international community to let Russian aid, or Russian soldiers, into the troubled region.

Amid renewed shelling on Sunday, separatists re-iterated the ceasefire proposal. However, rebel spokeswoman Elena Nikitina added that the Ukrainian government was "incapable of negotiating" and that talks could only begin if "the Ukrainian army is withdrawn" from the region.

Meanwhile, Ukraine military spokesman Andriy Lysenko told Reuters on Sunday that a truce offer should only be made by "raising white flags and by putting down guns...We have not seen these practical steps yet."

Aid mission on the cards?

Several thousand of Donetsk's one million residents have fled since the conflict between pro-Russian separatists and government troops began in April, and more than 1,300 people have died, according to the United Nations.

In a phone conversation late Saturday, Russian Foreign Minister Sergei Lavrov sought support from his US counterpart John Kerry for a Russian aid mission to the region.

The West has long accused Russia of amassing troops on the border with Ukraine, and of supplying arms to the rebels. On Friday, Ukraine claimed to have blocked a Russian military convoy disguised as an aid truck from crossing the border. Moscow denies it tried to send troops into Ukraine.

In a series of separate telephone calls on Saturday, US President Barack Obama, German Chancellor Angela Merkel and British Prime Minister David Cameron agreed that any intervention by Russia in Ukraine without Kyiv's permission would be seen as "illegal" and "unacceptable," the White House said.

In an earlier conversation, Ukraine President Petro Poroshenko had told Merkel he was ready to accept an international aid mission, but only if staff were unarmed and entered through government-controlled borders.

nm/tj (Reuters, AP, AFP, dpa)