KYIV, Ukraine -- In a new push for peace in eastern Ukraine, the leaders of France and Germany announced Thursday they were heading to Kyiv 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 fears 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 defences 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 Kyiv, 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.

"Our choice is diplomacy," Kerry said, making no mention of providing Ukraine with lethal military aid.

Fighting between Russia-backed separatists and Ukrainian government forces surged in January. The United Nations has sharply criticized both sides for indiscriminate shelling and urged a temporary truce to halt the fighting that has killed over 5,300 people since April.

At least three people were killed in overnight shelling in the rebel stronghold of Donetsk, local officials said Thursday. A Ukrainian military spokesman said five servicemen died and 29 others were wounded in fighting in the east in the last 24 hours.

France and Germany were hoping this time they can come up with a peace deal that both Ukraine and Russia can agree to.

French President Francois Hollande said he and German Chancellor Angela Merkel would travel to Kyiv on Thursday and then to Moscow the following day with a proposal "based on the territorial integrity of Ukraine."

In a sign of the importance of the initiative, this will be Merkel's first trip to Moscow since Ukraine's conflict broke out a year ago.

"It will not be said that France and Germany together have not tried everything, undertaken everything, to preserve the peace," Hollande said.

The French leader did not mention the U.S. in any context, saying the two European nations have special historic, cultural and economic ties with Russia. A senior French government official said the two leaders decided Wednesday night on the trip and did not consult American officials about it. The official was not authorized to be named, according to French policy.

Kerry, however, sought to cast the new European initiative as part of unified Western efforts to support Ukraine.

He said the visit to Kyiv by Merkel and Hollande "underscores that together the United States, France, Germany and the rest of our international partners stand united with Ukraine in calling on Russia to take the steps that I just outlined and to take those steps without delay."

In response, Putin's foreign affairs adviser Yuri Ushakov said Russia was "ready for a constructive conversation" aimed at stabilizing the situation, establishing a dialogue between the Ukrainian government and the rebels and rebuilding economic ties between eastern Ukraine and Kyiv.

Western diplomats said Putin gave French and Germans a nine-page peace plan, and that Hollande and Merkel are taking a repackaged version of that with them to Kyiv and Moscow. The diplomats said the European version drops the most objectionable elements of the Russian plan and adapts it to fit what Ukraine and Europe want, such as some autonomy for eastern regions with special protections for language, culture and local taxes.

Kerry came to Ukraine to show support for its embattled government -- bringing $16.4 million in new humanitarian aid -- as the Obama administration weighed sending arms to Kyiv to help it fight the heavily armed, Russian-backed separatists. President Barack Obama has opposed the idea of sending weapons to Ukraine but sources in his administration say this position could change.

Germany and other European nations remain fiercely opposed to sending arms to Ukraine.

German Foreign Minister Frank-Walter Steinmeier reiterated that stance Thursday, saying it would not improve the situation if "we now bring more weapons to the region."

"By throwing more weapons on the bonfire, I don't believe, unfortunately, that we will solve the problems in Ukraine," Danish Foreign Minister Martin Lidegaard said in Copenhagen, urging more sanctions against Russia to force the rebels back into peace negotiations.

Breedlove, the top NATO commander, warned Thursday that any move to give Ukraine lethal defensive weapons must take into account a possible angry reaction from Russia.

"(That) could trigger a more strident reaction from Russia," he said.

European Union foreign ministers will be talking Monday about increasing EU sanctions against Russia for its actions in Ukraine. Federica Mogherini, the EU's foreign policy chief, backed the French-German peace effort, saying "there is no military solution to the crisis in Ukraine."

In Brussels, NATO Secretary-General Jens Stoltenberg said the group's defence ministers on Thursday are expected to approve boosting the size of the alliance's Response Force from 13,000 to 30,000, in reaction to Russian actions in Ukraine.

Hollande appeared to offer an olive branch Thursday to Putin on the Russian leader's key demand: that Ukraine stay out of NATO.

"France is not favourable to Ukraine's entry into NATO, let us be clear," Hollande said. "For the Russians who are worried ... we have to settle this problem among Europeans. We are on the same continent."

Ushakov, Putin's aide, praised Hollande's comment, saying it reflects "a pragmatic approach, which the Kremlin welcomes."