German Chancellor Angela Merkel and French President Francois Hollande flew to Kiev Thursday with a new peace plan, in the biggest diplomatic push yet to halt an upsurge in deadly fighting in eastern Ukraine.
The two European leaders, who will present the plan to Russia Friday, arrived hot on the heels of US Secretary of State John Kerry, who said that Moscow held the cards to end the conflict.
The renewed efforts to end the 10-month conflict came as Washington weighs sending arms for Kiev's fight with pro-Russian separatists.
At least 21 people have been killed in fresh clashes in the past 24 hours, as the separatists attempt to push deeper into government-held territory.
Kerry threw Washington's weight behind the latest peace initiative, although Ukraine's premier said he was sceptical.
"President Putin can make the choices that could end this war," Kerry said after meeting Ukrainian leaders, voicing support for the "helpful" Franco-German plan to be put to the Russian leader on Friday.
"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 told journalists.
Kerry added that US President Barack Obama would "soon" decide on whether to arm Ukraine, but stressed his preference for a diplomatic solution.
- 'No proxy war' -
"We are not interested in a proxy war, our objective is to change Russia's behaviour and we'll consider all options that are available to us in coordination with our partners that will help us achieve a negotiated solution," Kerry said.
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, ahead of Obama holding a "very important" meeting with Merkel at the White House on Monday, Kerry said.
The new plan offers greater autonomy within Ukraine for the separatists, and across a larger area than agreed at the last truce deal in Minsk in September, Germany's Sueddeutsche Zeitung said.
Ukrainian President Petro Poroshenko had to understand this was his country's last chance to avoid military defeat and economic collapse, while Putin was warned to count on fresh European Union sanctions, the paper said, citing unnamed sources.
- NATO bolsters defences -
Speaking in Paris before heading to Kiev Hollande said that 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 Poroshenko on Thursday and Putin in Moscow on Friday, but warned that diplomacy "cannot go on indefinitely".
Despite the diplomatic momentum behind the new plan, Ukrainian Prime Minister Arseniy Yatsenyuk insisted Moscow stick to a September's Minsk agreement.
"To have a new deal, not to execute the previous one, seems to me a trap," Yatsenyuk told journalists.
"We urge Russia to implement and execute what was agreed, signed personally by president Putin."
In Brussels, NATO agreed 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.
The fighting in Ukraine has claimed more than 5,350 lives since April, including some 220 in just the past three weeks, according to the United Nations.
Renewed attempts at peace talks in the Belarussian capital Minsk over the weekend collapsed after rebel leaders stayed away.
NATO Secretary General Jens Stoltenberg said 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 announced six "command and control" units in eastern European nations to ensure that the new force could hit the ground running.
- Civilian deaths rise -  
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 the battleground railway hub of Debaltseve, which rebels are trying to encircle and capture, Kiev officials said.