In a new push for peace in eastern Ukraine, the leaders of France and Germany announced Thursday they were heading to Kiev and Moscow with a proposal to end the fighting. The surprise move appeared designed to head off U.S. considerations of giving Ukraine heavy weapons, something Europeans oppose out of fear of sparking an arms race.

The flurry of high-level diplomacy aimed to end the resurgent fighting in eastern Ukraine that is threatening European security.

In Moscow, President Vladimir Putin’s aide welcomed the new European initiative and said the Kremlin was ready for a constructive discussion. In Brussels, NATO foreign ministers prepared to boost the military alliance’s forces Thursday in response to the fighting and Russia’s increased military forcefulness.

Russia has vehemently denied backing the rebels with troops and weapons, while acknowledging that some Russians are fighting with the separatists. Western military experts scoff at the Russian denial. The top NATO commander, U.S. Air Force Gen. Philip Breedlove, said Thursday that Russia continues to supply the separatists with heavy, state-of-the-art weapons, air defenses and fighters.

Calling it a “very critical moment in our history,” Ukrainian President Petro Poroshenko warmly welcomed U.S. Secretary of State John Kerry to Kiev, the Ukrainian capital, on Thursday.

In a joint news conference after their talks, Kerry urged Russia to show its commitment to a peaceful, diplomatic solution to the conflict in eastern Ukraine by ceasing its military support for the separatists and bringing them to the negotiation table.

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This story is compiled with information from The Associated Press.