Heavy combat fire has rocked the outskirts of the strategic Ukrainian port city of Mariupol, just hours before talks open on a possible ceasefire.

A series of explosions were also heard in the main eastern rebel hub of Donetsk overnight, apparently coming from the city’s airport, an AFP journalist said.

Mariupol has become the latest flashpoint in the near five-month conflict in eastern Ukraine as pro-Russian rebels wage a counter-offensive that has seen swathes of land seized from government forces in just a matter of days.

Residents of the government-held city on the Sea of Azov have been digging in for fear of a major onslaught by the rebels, apparently backed by Russian troops and firepower.

AFP journalists reported heavy fire from the east of the city on Friday, with Ukrainian volunteer militias reporting attacks by insurgents on several checkpoints.

Ukrainian government forces had for weeks been besieging Donetsk, the biggest city in the largely Russian-speaking industrial east of the country in their battle to rout the rebels.

But the troops have now pulled back from positions around the city in the face of the insurgent advance.

The renewed fighting comes despite hopes that talks in the Belarussian capital Minsk later on Friday on a peace plan unveiled by Russian President Vladimir Putin will lead to a ceasefire.

Ukrainian President Petro Poroshenko said on Thursday the peace blueprint would be signed on Friday at the meeting brokered by the pan-European security body the OSCE.

The leaders of two pro-Russian rebel regions in Ukraine’s eastern industrial heartland also said they were ready to issue orders to silence the guns once a deal is done.

The push for peace follows a dramatic surge in tensions after NATO reported that Russia had secretly sent in troops and heavy weapons to support the insurgents in a new counter-offensive that has seen Kiev’s troops lose control of large areas of the southeast.

Russia denies it has any involvement in the conflict but NATO leaders are expected to announce a raft of fresh sanctions against Moscow if no peace deal is agreed.