Germany's prime minister Angela Merkel said on Saturday that sending arms to help Ukraine fight pro-Russian separatists would not solve the crisis there, drawing sharp rebukes from US politicians who accused Berlin of turning its back on an ally in distress.

The heated exchanges at a security conference in Munich pointed to cracks in the transatlantic consensus on how to confront Russian President Vladimir Putin over a deepening conflict in eastern Ukraine that has killed more than 5,000 people.

Ukraine's military said on Saturday that pro-Russian separatists had stepped up shelling of government forces and appeared to be amassing troops for new offensives on the key railway town of Debaltseve and the coastal city of Mariupol.

The rebel offensive has triggered a flurry of shuttle diplomacy, with Merkel and French President Francois Hollande jetting to Moscow on Friday to try to convince Putin to do a peace deal.

But European officials acknowledge that the Russian leader may have little incentive to negotiate now, preferring to sit back and watch as separatists seize more territory, undermining a ceasefire agreement clinched last September in the Belarus capital Minsk.

The German leader conceded in Munich that it was uncertain whether a Franco-German peace plan presented to Kiev and Moscow this week would succeed.

But she rejected the notion that sending weapons to Kiev, an idea being considered by US President Barack Obama, would help resolve the conflict.

"I understand the debate but I believe that more weapons will not lead to the progress Ukraine needs. I really doubt that," said the conservative German leader, who has led western efforts to try to resolve the crisis through negotiations and will travel to Washington on Sunday for talks with Obama.

BLANKETS VS TANKS

US senators Lyndsey Graham and John McCain, two Republican hawks who attended the conference, were withering in their criticism of the German stance, which is supported by other big European countries like France.

"At the end of the day, to our European friends, this is not working," Graham said of Merkel's diplomatic efforts. "You can go to Moscow until you turn blue in the face. Stand up to what is clearly a lie and a danger."

McCain added: "The Ukrainians are being slaughtered and we're sending them blankets and meals. Blankets don't do well against Russian tanks."

US Vice President Joe Biden, also speaking in Munich, tried to play down differences with Europe, saying he and Obama agreed that no efforts should be spared to resolve the conflict peacefully.

But he made clear that Washington stood ready to provide Ukraine with the means to defend itself, saying: "Too many times President Putin has promised peace and delivered tanks, troops and weapons."

A senior State Department official at the Munich talks also denied there was a rift between Washington and the Europeans.

"We will see where we end up policy-wise, but as of now with the diplomatic approach and with the situation regarding the provision of weapons we are all on the same place," the official said.

EMOTIONAL PLEA

Russia's annexation of the Crimean peninsula in March last year and evidence that it is supporting separatist forces in the east of the country, which the Kremlin denies, have driven Moscow's relations with the West to a post-Cold War low.

The EU and United States have imposed a series of sanctions against Moscow that have contributed to a sharp downturn in the Russian economy.

Merkel and her allies in Europe want to continue to punish Russia by tightening the economic screws. Obama faces pressure from members of Congress to do more.

The senior State Department official said the Franco-German initiative was based on a peace plan reached in Minsk last September but added it contained more details on implementation and timing of the deal.

The September Minsk peace plan called for a ceasefire as part of a 12-point blueprint for peace. It also called for tighter control of the joint Russia-Ukraine border, through which Kiev says Moscow is funnelling fighters and equipment, and the freeing of prisoners held by the two sides.

Ukrainian President Petro Poroshenko said on Saturday the deal with Moscow could only work if all 12 elements of the Minsk agreement, sealed last September in Belarus, were respected.

Since the agreement, however, pro-Russian separatists have made huge territorial gains in eastern Ukraine, fuelling doubts over whether they will respect it in its current form.

In an emotional plea for support in Munich, Poroshenko listed the number of troops and civilians that had been killed since the crisis started and held up red passports of Russian soldiers he said had been found fighting in Ukraine.

"We are an independent nation and we have a right to defend our people," he said, calling for political, economic and military support. Calling himself a "president of peace", he made clear that Kiev only wanted defensive weaponry.

He received strong backing from Lithuanian President Dalia Grybauskaite, who said Ukraine should be supported "with all means necessary to defend, not to attack, to defend its people and its territory."

Russian Foreign Minister Sergei Lavrov, also in Munich, said there were "good grounds for optimism" that the talks between Merkel, Putin and Hollande could yield a deal.

But Lavrov also delivered a diatribe against the West. He accused Europe and the United States of supporting a "coup d'etat" against deposed Ukrainian leader Viktor Yanukovich, a Moscow ally, a year ago and turning a blind eye to nationalists he said were intent on ethnic cleansing in eastern Ukraine.

Hollande, speaking to reporters in the city of Tulle in central France, cast the talks with Putin as a last-ditch effort to avert full-blown conflict.

The French leader, Merkel, Poroshenko and Putin are due to hold a call on Sunday, before the chancellor travels to Washington.

"If we don't manage to find not just a compromise but a lasting peace agreement, we know perfectly well what the scenario will be. It has a name, it's called war," Hollande said.