The leaders of Ukraine, Russia, France and Germany were to hold peace talks Wednesday aimed at halting a 10-month war in Ukraine where dozens were killed in the latest fighting.

In the run-up to the summit, the climax of a frantic diplomatic push to prevent the worst East-West crisis since the end of the Cold War from escalating, Russia expressed optimism.

"Experts are working, there is noticeable progress," Foreign Minister Sergei Lavrov said, although he signalled there would be no flexibility on the crucial negotiating demand from Ukraine that it be given back control of its border with Russia.

Ukrainian President Petro Poroshenko warned he could "introduce martial law throughout Ukraine" if the talks in the Belarussian capital Minsk failed to stop the war with the pro-Russian separatists.

"I will not hesitate with this decision, if the actions of the aggressor lead to further escalation," he told a cabinet meeting.

Martial law would mark a grave escalation of the crisis, freeing up military resources for the fight in the east but also likely leading to the cutting off of foreign investments, including a vital loan from the International Monetary Fund.

- 'One voice' -

The pro-Western Ukrainian leader said he, French President Francois Hollande and German Chancellor Angela Merkel would speak "with one voice" to Russian President Vladimir Putin, whom they accuse of backing the separatist rebellion.

"The key position is that we need an unconditional ceasefire," he said.

US President Barack Obama has warned Putin that Russia would be made to pay if the talks fail.

A French presidency source said Hollande and Merkel would "try everything" to achieve peace but that hours before the start of the summit at about 1530 GMT there were "quite a lot of problems still to be resolved".

Merkel's spokesman said the summit offered "a glimmer of hope, nothing more".

"It is uncertain whether an outcome can be reached, but despite all the uncertainty, it is worth trying in the interest of the suffering people in eastern Ukraine," said Steffen Seibert.

The bloodletting on both sides has been relentless in recent weeks as pro-Russian separatists have pushed a new offensive and Kiev forces have counter attacked.

At least 50 people were reported killed in the last 24 hours, including 16 in a devastating rocket attack on Kramatorsk, the Ukrainian government's eastern military headquarters and administrative hub.

In the rebel stronghold of Donetsk, city officials said that 11 people had been killed, including nine in a mortar strike on the city early Wednesday.

Rebels, who rarely announce military casualties, said they had lost seven fighters.

Poroshenko, who was scheduled to join a European Union summit in Brussels on Thursday to discuss the conflict, will in Minsk come face-to-face with Putin for the first time since October.

If the talks fail, Obama has warned that Washington may decide to start providing lethal weapons to Ukraine, a step many European leaders oppose for fear of getting drawn into open conflict with Russia.

On Tuesday, Obama spoke to Putin by phone and sought to pressure him to rein in the rebels, who have close political links to Moscow, and embrace the chance for peace.

"If Russia continues its aggressive actions in Ukraine, including by sending troops, weapons, and financing to support the separatists, the costs for Russia will rise," the White House said.

- New proposals, old plans -

The plan to be discussed is based largely on repeatedly broken peace deal between Kiev and the rebels in September. The hope is that, at minimum, a ceasefire to halt fighting that has killed hundreds of civilians in recent weeks can be agreed upon in Minsk.

A key sticking point is whether a new deal will extend rebel control over some 500-square kilometres (200-square miles) of territory seized over the past month.

As leaders converged on Minsk, fighting raged on the ground with both sides trying to strengthen their hand at the negotiating table.

Insurgent fighters have been battling for weeks to take the rail hub of Debaltseve, while Ukrainian forces on Tuesday captured ground around the port city of Mariupol.

Kiev is desperate to get Putin -- who has watched Western sanctions and low oil prices batter the Russian economy -- to put his signature on a deal. But the former KGB spy has consistently told Ukraine's government it needs to reach an agreement with the rebels, not with him.

Moscow is pushing for the separatist-held territories to be granted a large degree of autonomy, while Ukraine is demanding it get control back over some 400 kilometres (250 miles) of its border with Russia.

Kiev and the West accuse Putin of pouring soldiers and troops across the border into Ukraine to spearhead the insurgency, but Moscow denies it is behind the fighting.

Lavrov said that Moscow did not support allowing Ukraine to regain control of its frontier.

"While there is fighting, while many other issues remain unsolved, this will be unrealistic, in my opinion," Lavrov told reporters.