A peace proposal for Ukraine edged toward a possible breakthrough as the leaders of Germany, France, Russia and Ukraine agreed Sunday to a joint summit alongside representatives of the pro-Moscow separatists who have waged a bloody campaign in the Ukrainian east.

The four leaders agreed to the proposed summit — scheduled for Wednesday in Minsk, the capital of Belarus — during a four-way phone call Sunday. The success of the summit, though, still appeared to hinge on further diplomatic talks Monday in Berlin, aimed at laying the groundwork for a “comprehensive settlement” of the crisis in Ukraine, where fighting has steadily worsened.

The German government announced the summit plans on the heels of whirlwind visits last week to Moscow and Kiev, Ukraine, by German Chancellor Angela Merkel and French President François Hollande, who have launched a high-stakes diplomatic push to end an escalation of the fighting and resolve a growing standoff between the West and Russia. European and U.S. governments have accused Moscow of subterfuge for denying its involvement in Ukraine even as the West cites conclusive intelligence indicating that Russian weapons and disguised troops are fighting in eastern Ukraine.

Yet diplomats cautioned that stumbling blocks remain , including the highly charged issue of whether recent land gains made by separatists would be recognized as part of a cease-fire agreement. And there is skepticism that Moscow and the separatists would adhere to a deal . A previous accord reached in Minsk last September — which aimed to create a demilitarized zone and deliver more autonomy for the rebel-held lands — was routinely violated before largely breaking down in recent weeks.

“Even after this weekend of intense negotiations, we are far away from a political solution of the Ukraine conflict,” German Foreign Minister Frank-Walter Steinmeier said in remarks on the last day of a major security summit here.

Speaking in the Russian resort town of Sochi, Russian President Vladimir Putin offered a qualified confirmation of the summit plans, saying the meeting would go ahead “if, by then, we have agreed upon a number of positions that were a subject of our intense discussions recently.”

The summit plans came as the Ukraine crisis dominated the security conference in Munich, which drew top policymakers from all sides of the conflict. On Sunday, Sen. John McCain (R-Ariz.) underscored the divisions over how to handle the crisis by reiterating calls for the West to supply Kiev with defensive arms. A host of Europeans, led by Merkel, have denounced the notion as a way to ensure that the conflict escalates.

“The question is, how long can Putin sustain a war he tells his people is not happening?” McCain asked in a speech. “That’s why we must provide defensive arms to Ukraine.”

But Secretary of State John F. Kerry seemed to support the German view, saying that there is no military solution to the fighting and denying any rift between the United States and Europe over how to respond to Russia’s support of the separatists.

“There is no division. There is no split,” he said. “I keep hearing people trying to create one. We are working closely together. We all agree this challenge will not end through military force.”

“I assure you,” he added, “no matter what, we will stand together in support of Ukraine and in defense of the common understanding that international borders must not, cannot, be changed by force, in Europe or anywhere else.”

Later, the State Department spokeswoman, Jen Psaki, issued a statement endorsing the Minsk talks. “We continue to support the ongoing diplomatic efforts by our European colleagues and remain in lockstep that any agreement must respect the sovereignty and territorial integrity of Ukraine,” she said.

Thousands of Ukrainian civilians are trapped between the battle lines in the renewed fighting.

She said the United States is concerned about reports of fierce fighting in the east, and of new Russian convoys there.

The Obama administration has been reluctant to send lethal weaponry to Kiev, but it is under pressure from Congress. McCain, the organizer of a bilateral delegation of 15 senators and House members attending the Munich Security Conference, argued that the West must help Ukraine defend itself and raise the war’s cost to Putin.

“There is little reason to be optimistic about a new Minsk agreement since Russia has so blatantly violated the last one by sending additional heavy weaponry and troops into Ukraine,” Rep. Adam B. Schiff (D-Calif.), the top Democrat on the House Intelligence Committee, said in a statement Sunday in Washington. “Any agreement must not allow the Russian-backed rebels to benefit from breach of the last accord — they must withdraw to the previously agreed upon lines. I continue to believe that providing defensive weapons to allow Ukraine to protect itself from Russian aggression will be necessary to deter any further violations of Ukrainian sovereignty.”

The European Union said Monday that it planned to expand its sanctions against Russians and Ukrainian separatists because of the latest developments in Eastern Ukraine, but would put off the action until Feb. 16, to give the negotiations a chance to proceed.

The peace proposal being hashed out ahead of Wednesday’s summit is likely to reiterate at least some parts of the earlier agreement, which effectively recognized rebel control of significant parts of Ukraine’s industrial east. But since then, the rebels have made fresh gains, and analysts say the Russians and the rebels have little incentive to concede those land advances as part of any deal.

“The principal problem, and the principal roadblock, will be how much autonomy the separatist regions will have and what kind of territory they will eventually encompass,” said Vladimir Frolov, a foreign policy analyst based in Moscow and a former diplomat.

Yet Ukrainian President Petro Poroshenko is in a tight fix at home. He expended significant political capital by granting concessions to the rebels last time around, making new concessions difficult to sell domestically. But his army is no match for rebel outfits that the West says include Russian troops and weapons.

“The Russians have the upper hand,” said Stefan Meister, an analyst at the German Council on Foreign Relations in Berlin.

Stephanie Kirchner in Berlin, Karoun Demirjian in Moscow and Niraj Chokshi in Washington contributed to this report.