UPDATE 2-Ukraine reinforces defence of key port, shelling rocks Donetsk
* Rebels advancing on Azov Sea port of Mariupol - reports
* Fresh shelling of rebel-held Donetsk kills at least one
* Ceasefire may be agreed Friday in Minsk (Adds renewed fighting in city of Donetsk)
By Aleksandar Vasovic and Gabriela Baczynska
MARIUPOL/DONETSK, Ukraine, Sept 4 (Reuters) - Ukraine reinforced its defences at the port of Mariupol on the Sea of Azov on Thursday in anticipation of a rebel attack amid reports that pro-Russian separatists were advancing on the city with tanks and artillery.
Further north, in the rebel stronghold of Donetsk, renewed shelling killed at least one woman and blew huge holes in residential buildings, a day before talks between Kiev and the separatists on a possible ceasefire.
A Ukrainian soldier told Reuters he had seen the separatists advancing on Mariupol, a city of about 500,000, with tanks, armoured personnel carriers and artillery. His comments could not be independently confirmed.
A Reuters witness heard about a dozen blasts and saw plumes of black smoke rising a few km (miles) to the east of Mariupol. He also saw an armoured personnel carrier conveying a wounded soldier into the city.
A Ukrainian officer said the rebels had been taking over army checkpoints seven km (five miles) away, adding: "There will be an attack, we are waiting for them here (in Mariupol)."
Mariupol is Ukraine's largest port on the Sea of Azov and stands on the way to Russian-annexed Crimea. It has become a focus of Ukrainian concern after rebels broke away from their main strongholds further north and took the nearby coastal town of Novoazovsk, to the east of Mariupol, in late August.
Kostyantyn Batozsky, deputy governor of the Donetsk region where Mariupol is located, said on his Facebook page that Ukrainian forces had destroyed four tanks belonging to advancing pro-Russian separatists.
The governor of Donetsk region, Serhiy Taruta, vowed to defend Mariupol from the rebels during a visit to frontline troops but also said he hoped tentative plans for a ceasefire in the region on Friday would be confirmed.
"We are fighting to repel the DNR (the rebels' self-proclaimed Donetsk People's Republic), Russia and whoever else wants to come here. We are defending the city," he said.
"We are hoping for a ceasefire, talks and resolution of all unresolved issues within a sovereign Ukraine," Taruta added.
Earlier, while attending a NATO summit in Wales, Ukraine's President Petro Poroshenko said he expected envoys due to meet in Minsk on Friday to agree a ceasefire that would pave the way for implementation of a peace plan for his country.
In Donetsk, with a pre-war population of about one million, a Reuters correspondent saw pools of blood in debris from damaged apartment blocks on Thursday in the north of the city.
At least five buildings had been hit, including a church, and their windows had been blown out. Walls had been strafed by shrapnel.
Local residents covered a dead woman dressed in a bright pink shirt with a sheet of metal. Observers from the Organization for Security and Cooperation in Europe (OSCE) rights and security group were surveying the scene.
Envoys of the OSCE, Russia and Ukraine are expected to approve a peace plan for Ukraine at their meeting in Minsk on Friday, though both sides in the conflict remain sceptical about the chances of any ceasefire holding for very long. (Writing by Gareth Jones, Editing by Angus MacSwan and Dominic Evans)