The leaders of Germany and France on Thursday announced a surprise diplomatic bid to end the conflict in Ukraine, working to forestall White House deliberations about arming government forces amid fears that the war could quickly spiral out of control.

The new peace effort came as civilian and military casualties have mounted by the hundreds in recent weeks. German Chancellor Angela Merkel and French President François Hollande said they would fly to Moscow on Friday to meet Russian President Vladimir Putin to try to strike a deal. The unusual journey underscored the high stakes in the effort to quell the bloodiest European conflict since the Balkan wars of the 1990s.

But Western officials noted deep uncertainty about whether the diplomacy would yield results, as Russia has steadfastly maintained that it is not a party to the conflict. Ukraine and its Western allies have said the Kremlin is fueling the war with weaponry, supplies and soldiers.

Secretary of State John F. Kerry on Thursday joined in the efforts in a visit to the Ukrainian capital, arriving just hours ahead of Merkel and Hollande.

Despite calls in Washington for the United States to start arming Ukraine with defensive weaponry, Kerry said the White House is still reviewing its options.

President Obama will make a decision on arms “soon” after talks with advisers, Kerry said.

“We want a diplomatic resolution,” he said. “But we cannot close our eyes to tanks that are crossing the border from Russia and coming into Ukraine. We can’t close our eyes to Russian fighters in unmarked uniforms crossing the border and leading individual companies of so-called separatists in battle.”

Kerry urged Russia to pull back weapons and troops to restore the tattered cease-fire. He also called for a sealed border between rebel-held eastern Ukraine and Russia.

Russian diplomats said Thursday that they would view any U.S. move to arm the Ukrainians as a direct threat to their nation’s security.

More than 5,300 people have died in Ukraine since hostilities began in April, according to U.N. estimates. Rebels have taken hundreds of square miles of territory since a September deal to halt the fighting and freeze the territorial land grabs. At least eight people died in the past 24 hours.

Rebel leaders warned this week that they would mobilize vast forces against Ukrainian troops in the eastern part of the country. Russia, meanwhile, announced large-scale military exercises in recent days; such a step came ahead of a previous escalation in August.

The sudden diplomatic effort — which will bring Merkel to Moscow for the first time since relations soured with Russia nearly a year ago — appeared to be a last-ditch measure to halt the conflict. Merkel, a Russian speaker from formerly Communist East Germany, was once the Western leader considered best able to bargain with Putin. But she broke with him this year after the escalations in Ukraine and has been reluctant to hand him the public relations coup of a visit to Moscow, analysts say.

Putin sent his Ukrainian counterpart a letter in recent days with proposals to halt the fighting, his top foreign policy aide, Yuri Ushakov, told reporters in Moscow on Thursday.

Western officials who had been briefed on Putin’s proposals said that they amounted to an attempt at a new quasi-independent ­Russian-backed statelet similar to Transnistria in Moldova and Abkhazia in Georgia — a step that would be distasteful to the West and politically impossible in Kiev.

“It is not a peace plan. It is a road map to creating a new Transnistria or Abkhazia in Ukraine,” a Western diplomat told reporters under ground rules of anonymity. “It is a cynical effort to get out of all the commitments made in Minsk,” where the September cease-fire was signed.

Merkel and Hollande were going to present their plan on Friday, after consulting with Ukraine’s Western-allied president, Petro Poroshenko, in Kiev.

In Paris, Hollande said that he and Merkel plan “a new proposal to solve the conflict which will be based on Ukraine’s territorial integrity.” But he said he did not favor NATO membership for Ukraine, a concession to Russian officials who have said membership would be a major threat.

Hollande warned that “total war” could loom in Ukraine if fighting is unchecked.

The Obama administration has limited its assistance to Ukraine. Much has been humanitarian, though the United States has provided some military equipment, such as night-vision goggles.

“The risk is we’re doing too little, not doing too much,” said Rep. Adam Schiff (Calif.), the top Democrat on the House Intelligence Committee, who is gathering signatures on a letter urging the administration to send Ukraine defensive weaponry.

In addition to Schiff’s initiative, leading think tanks and former senior Obama administration officials have called for arming Ukraine. Eleven members of the Senate Armed Services Committee from both parties — including the chairman, Sen. John McCain (R-Ariz.), and the ranking Democrat, Sen. Jack Reed (R.I.) — added their voices at a news conference Thursday. Reed noted that Congress had overwhelmingly supported “lethal and non-lethal military assistance” for Ukraine in the 2015 National Defense Authorization Act.

Kerry announced in Kiev that the United States would provide $16 million in humanitarian aid to buy blankets, repair homes, obtain wheelchairs and provide counseling for war victims.

The prospect of arming Ukraine has drawn concerns not just from Russia but also from close U.S. allies in Europe. Germany and France have said they have no plans to do so.

“The German government is very clear in its statements. One should not pour oil into the fire,” said Markus Kaim, a security analyst at the German Institute for International and Security Affairs.

One possibility, Kaim said, would be a new package that would allow both sides to save face: for example, by offering a European Union trade deal in return for Russia’s enforcing the peace plan.

Any move toward arming Ukraine would almost certainly invite a reaction from Moscow, which is otherwise aligned with Washington on such issues as the Iran nuclear talks and the need to stop Islamic State militants in Syria and Iraq.

“Even if we try to do this covertly, there’s a risk that it will boomerang in other areas of the U.S.-Russia relationship,” said Andrew C. Kuchins, director of the Russia and Eurasia program at the Center for Strategic and International Studies.

NATO defense ministers on Thursday agreed to significantly bolster quick-deploying forces that could react to any threat to NATO territory from Russia. One large-scale response group will increase from 13,000 to 30,000. Another quick-reaction ground force of 5,000 troops will be created.

E.U. leaders also said they plan to discuss further sanctions against Russia and the rebels at a meeting next week.

Karoun Demirjian in Soledar, Ukraine, Anthony Faiola in Munich, Stephanie Kirchner in Berlin and Karen DeYoung in Washington contributed to this report. Birnbaum reported from Moscow.