Amid fears that accelerated fighting in eastern Ukraine could endanger Europe’s overall security, the leaders of Germany and France met Thursday with Ukrainian President Petro Poroshenko in a hastily arranged mission.

German Chancellor Angela Merkel and French President Francois Hollande carried a peace plan that reportedly incorporates proposals from Russia. They are scheduled to meet today with Russian President Vladimir Putin in Moscow.

The trip by the leaders of Western Europe’s two most populous nations came as concerns rose about whether the U.S. would send weapons to Ukraine and as NATO formed a quick-reaction force of 5,000 soldiers in response to Russia’s increased military muscle-flexing.

Hollande and Merkel offered no comment as they left the presidential officest. Details of their plan remained unclear, although Poroshenko said at the start of the meeting that it raised hopes for calling a quick cease-fire.

Foreign Minister Pavlo Klimkin said on Twitter that the leaders discussed “steps so that the Minsk agreement can start working,” referring to a pact reached in September that called for a cease-fire, the pullback of heavy weaponry by both sides, international monitoring of the Ukraine-Russia border and a degree of autonomy for the east.

The high-level diplomacy came as resurgent fighting killed eight more people in eastern Ukraine. More than 5,300 people have been killed since the fighting started in April.

The fighting in eastern Ukraine between Russia-backed separatist rebels and Ukrainian forces has intensified sharply over the past two weeks. Russia vehemently denies that it is providing equipment or troops for the insurgents, but U.S. Secretary of State John Kerry on Thursday was the latest in a line of Western officials to sharply reject that denial.

“Let there be no doubt about who is blocking the prospects for peace here,” Kerry said in Kiev, where he met with Poroshenko hours before the Hollande and Merkel.

In Moscow, a Putin aide welcomed the new European initiative and said the Kremlin was ready for a constructive discussion. NATO defense ministers in Brussels, however, boosted the alliance’s rapid response forces because of the fighting in Ukraine and Russia’s increased military muscle-flexing.

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.

Kerry, speaking to reporters after the talks with Poroshenko, 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.

At a later news conference with Ukrainian Prime Minister Arseniy Yatsenyuk, Kerry said President Barack Obama “is reviewing all his options; among those options obviously is the possibility of providing defensive systems to Ukraine.”

“We are not interested in a proxy war. Our objective is to change Russia’s behavior,” he said.

Nonetheless, U.S. lethal aid to Ukraine would likely intensify the conflict and further polarize the West and Russia.

In Moscow, Putin 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 Kiev. He said the Kremlin expects that Merkel and Hollande had taken Putin’s own peace proposals into account.

Western diplomats said Putin gave the French and Germans a nine-page peace plan, and that Hollande and Merkel took a repackaged version with them. The diplomats said the European version drops the most objectionable elements of the Russian plan but keeps elements such as a degree of autonomy for eastern Ukraine and special protections for culture in the largely Russian-speaking region.

Kerry said the French and German foreign ministers had informed Washington about the Russian proposal, but he didn’t have all the details. He brought $16.4 million in new humanitarian aid to Ukraine as the Obama administration weighed sending arms to help the beleaguered Kiev government fight the heavily armed separatists.

Obama has opposed sending weapons, but some in his administration have said hat position could change.

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

“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 talks.