Ukrainian forces pounded pro-Russian separatists to the east of the vital strategic port of Mariupol on Friday, just hours before ceasefire talks were due to begin in the Belarusian capital of Minsk.

Ukraine said its forces are trying to repel a major push by separatists to seize control of Mariupol, a strategic port city with a population of around 500,000 on the Sea of Azov crucial for its steel exports.

The capture of Mariupol would complete a land corridor to Crimea and leave Russia in control of the Azov Sea coastline.

“Our artillery has come and is being deployed against the (pro-Russian) rebels,” the mayor of Mariupol, Yuri Khotlubey, told Ukraine’s 112 TV channel.

Pro-Russian separatists told the Interfax news agency that about 50 Ukrainian soldiers had been killed or wounded in fighting near Mariupol on Thursday, and that three had been taken captive. This report could not be independently confirmed.

Mariupol became a major focus of concern for Ukraine after the rebels broke away from their main strongholds further north in late August - backed, Kiev says, by Russian regular forces – and took the nearby coastal town of Novoazovsk.

‘Careful optimism’ ahead of ceasefire talks

The shelling occurred shortly before representatives from Ukraine, the pro-Russian separatist leadership, Russia, and Europe’s OSCE security watchdog were expected to meet in Minsk to negotiate a ceasefire and “stage-by-stage peace plan” for the country.

On Thursday, Ukrainian President Petro Poroshenko expressed his “careful optimism” about the meeting, saying that he was prepared to order a ceasefire in eastern Ukraine if a deal was signed.

“Look, Ukraine is fighting for peace,” Poroshenko told a news conference, speaking in English. “It’s Ukraine which pays the highest price every single day, losing lives of soldiers, innocent civilians.”

Pro-Russian separatists also said that they were ready to declare a truce if an agreement was reached on a political settlement for the mostly Russian-speaking region.