Munich: Germany’s Angela Merkel warned on Saturday that sending arms to help Ukraine fight pro-Russian separatists would not solve the crisis there, drawing a sharp rebuke from a leading U.S. senator who accused Berlin of turning its back on an ally in distress.

The heated exchange at a security conference in Munich pointed to the fragility of 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.

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.

 A recent 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 say the Russian leader may have little incentive to negotiate now, preferring to sit back and watch the separatists make territorial gains in Ukraine that have made a mockery of a prior ceasefire agreement clinched last September in Minsk, Belarus.

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

The German leader conceded in Munich, after returning home from Moscow in the dead of night, that it was uncertain whether a Franco-German peace plan presented to Kiev and Moscow this week would succeed.

But she flatly rejected the idea that sending weapons to Kiev, an idea being considered by U.S. 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 a W

Western initiative to resolve the crisis through negotiations.

“The problem is that I can’t envision any situation in which a better-equipped Ukraine military would convince President Putin that he could lose militarily,” Merkel added.