Heavy shelling has pounded the centre of Ukraine's separatist-held city of Donetsk, according to witnesses.

People poured out of their offices onto the stairwell of the city's main administration building after loud explosions nearby triggered an evacuation warning, reports said.

Donetsk has been surrounded for several weeks by Ukrainian forces battling pro-Russian rebels.

The shelling follows the resignation of pro-Russian separatist leader Valery Bolotov, head of the self-proclaimed Luhansk People's Republic.

Mr Bolotov said he had been injured and could no longer continue his role. His resignation came as a Russian aid convoy resumed its journey toward Ukraine, taking the road leading south towards rebel-held Luhansk.

The convoy of roughly 262 vehicles had been parked at a military depot in the southern Russian city of Voronezh since late on Tuesday.

There has been confusion and disagreement over how and where the aid could be delivered to Ukraine, where government troops are battling pro-Russia separatists.

By sending the convoy south, Russia appeared intent on not abiding by a tentative agreement to deliver aid to a border checkpoint in the Kharkiv region.

It had been hoped that the convoy would arrive in the Kharkiv region, so that the Red Cross could inspect the convoy.

Instead, the route taken by the convoy leads directly toward a border crossing controlled by pro-Russian rebels in the Luhansk region.

"Direct invasion of Ukrainian territory" 

Ukrainian President Petro Poroshenko has accused Moscow of planning a "direct invasion of Ukrainian territory under the guise of delivering humanitarian aid."

Moscow has insisted it coordinated the dispatch of the goods - including baby food, canned meat and sleeping bags - with Red Cross officials.

Red Cross spokeswoman Anastasia Isyuk said talks between the organisation, Ukraine and Russia were continuing. But she could not confirm where the Russian convoy was headed.

"The plans keep changing, the discussions are going ahead and we will not confirm for sure until we know an agreement has been reached," Ms Isyuk said in Geneva.

Luhansk, where Mr Bolotov had declared himself "mayor", has been the scene of intense fighting between Ukrainian forces and separatists.

Mr Bolotov said Igor Plotnitsky, defence minister of the Luhansk People's Republic, would take over from him.

His resignation means that both the main separatist entities, in the Luhansk and Donetsk regions of eastern Ukraine, are having leadership changes.

On August 7th, Aleksander Borodai, head of the self-proclaimed Donetsk People's Republic, announced he was stepping down. Vladimir Putin said on Thursday that Russia would do its utmost to stop the bloodshed in Ukraine.

During a visit to Crimea, Mr Putin also said that Russia should not "fence itself off from the outside world" despite a plunge in East-West relations.

As well as the shelling in Donetsk, fighting has killed at least 22 residents in the besieged rebel-held bastion of Luhansk over the past 24 hours.

Fighting in eastern Ukraine has intensified in recent weeks, with United Nations officials saying there has been a spike in the number of deaths.

Some 2,086 people have died since the conflict began in mid-April, and more than half of them in the past fortnight, the UN said.