German and French leaders scrambled to Kiev and Moscow on Thursday with a new peace plan to stem an upsurge in deadly fighting in eastern Ukraine.
Chancellor Angela Merkel and President Francois Hollande were due in Kiev after a visit by US Secretary of State John Kerry as Washington weighed sending arms for Kiev's fight with pro-Russian rebels.
As the European leaders set out on the biggest peace push so far to end the near 10-month conflict, at least 21 people were killed in fresh clashes as the separatists attempt to push deeper into government-held territory.
"We talked about the largest threat that Ukraine faces today and that is Russia's continued aggression in the east," Kerry said after talks with Ukraine's leaders.
"There must be an immediate commitment now to a real ceasefire, which is not just a piece of paper with words, but which is followed by specific actions," Kerry said.
"We want a diplomatic resolution, but we cannot close our eyes to tanks that are crossing the border from Russia," he said, emphasising that "we are not seeking conflict with Russia".
In Paris, Hollande said he and Merkel would "propose a new solution to the conflict based on the territorial integrity of Ukraine."
He said the plan would be discussed with Ukrainian President Petro Poroshenko on Thursday and Russia's Vladimir Putin in Moscow on Friday, but warned that diplomacy "cannot go on indefinitely".
In Brussels, NATO was set to agree a major boost to the alliance's defences near its Russian borders, including six command centres and a quick-reaction spearhead force of 5,000 troops in response to the Ukraine conflict.
Despite growing talk of Washington arming Ukrainian forces, a State Department official said only that Kerry was to unveil $16 million (14 million euros) in fresh US humanitarian aid.
Russia, itself accused of arming the separatists, warned that any US move to send weapons to Ukraine would cause "colossal damage" to ties, foreign ministry spokesman Alexander Lukashevich said.
Kerry is to meet Russian Foreign Minister Sergei Lavrov at a security conference in Munich on Saturday after the three-way Moscow meeting.
Ukraine and its Western allies accuse Moscow of sending thousands of regular army troops and weapons to support the rebels who launched an uprising against Kiev in April.
And while Moscow has repeatedly denied the allegations, the separatists are equipped with the advanced weaponry of a regular army.
The fighting has claimed more than 5,350 lives since April, including some 220 in just the past three weeks, according to the United Nations.
Weekend peace talks in Minsk -- where a now-tattered truce was agreed in September -- collapsed after rebel leaders stayed away.
As the bloodletting has intensified, Washington has been reviewing its position to see if it should bow to a growing clamour to send hi-tech weapons to Kiev.
"We continue to evaluate as the situation on the ground changes the security needs of the Ukrainians... But no decisions have been made," a US official said.
NATO Secretary General Jens Stoltenberg said ahead of a meeting of defence ministers that the move to boost troops in eastern Europe was in response to Moscow's "aggressive actions," but nevertheless purely defensive.
In a move that is likely to irk Moscow, NATO is to decide on six "command and control" units in eastern European nations to ensure that the new force could hit the ground running.
Washington has so far provided only non-lethal assistance to Ukraine, including flak jackets, medical supplies, radios and night-vision goggles, because it fears becoming embroiled in a proxy war with Russia.
But the failure of economic sanctions to force Russia to halt what the West sees as Moscow's military support for the separatists has prompted a second look at the option.
In the rebel bastion of Donetsk, shelling killed eight civilians and wounded 33 more over the previous 24 hours, the insurgent-controlled city hall said.
Kiev-loyal authorities said six civilians had been killed around Donetsk, and military spokesman Vladyslav Seleznyov said that heavy shelling killed five Ukrainian soldiers and wounded 29 around the region.
Another two civilians died in battleground railway hub Debaltseve, which rebels are trying to encircle and capture, Kiev officials said.