Mr. McCain said that unless the West beefed up its support to Ukraine, Mr. Putin could next seize the port of Mariupol in eastern Ukraine with a view to building a land bridge from Russia proper to Crimea, which the Kremlin annexed last March. “I can assure you that he will not stop until he has to pay a much higher price,” Mr. McCain said.

After her appearance, Ms. Merkel met with Vice President Joseph R. Biden Jr. and President Petro O. Poroshenko of Ukraine. Mr. McCain, after meeting Mr. Poroshenko, said, “It’s safe to say that he’s not overly optimistic about this negotiation.”

Providing new details about the German and French initiative, a senior State Department official said Saturday that it was broadly consistent with the Minsk agreement but specified the steps necessary to implement a political settlement and the sequence and timing for carrying them out. The plan, when compared to the Minsk agreement, also appears to envision a wider buffer zone between Ukrainian forces and the separatists.

Ms. Merkel, Mr. Hollande, Mr. Putin and Mr. Poroshenko plan to speak Sunday to discuss the initiative.

“I don’t think we know yet how successful this effort will be,” said the State Department official who asked not to be identified under the agency’s procedures for briefing reporters. Mr. Biden said in his speech that while he had appeared before the conference six years before to urge that American-Russian relations be “reset,” that period was over; the West was now being tested by Russia’s actions and needed to change its focus.

“The Ukrainian people have a right to defend themselves,” said Mr. Biden, who stopped short of saying that the United States would provide lethal weapons.

Mr. Poroshenko appealed in his speech for military assistance, arguing that it would encourage Russia to accept a political solution. “The stronger our defense, the more convincing is our diplomatic voice.” And he strongly rejected the idea that additional territory concessions should be made in return for a new agreement.