One Obama administration official on Saturday cautioned that it is unlikely that a deal is imminent, and noted the difference in tone between the statements issued by the United States and by Russia on what was said in the telephone call between the two presidents. (The White House stressed possible diplomatic movement, while the Kremlin stressed Mr. Putin’s complaints about “extremists” in Ukraine.)Russia’s solution to the impasse over Ukraine emphasizes a federation, allowing for greater autonomy for eastern and southern Ukraine, with their heavy concentration of ethnic Russians. The stress Moscow places on the federation concept is seen partly as an attempt to ensure that Ukraine does not coalesce into a strong pro-European, anti-Russian country right next door.

Mr. Lavrov rejected as “absolutely unacceptable” the formula devised by Western officials, whereby Russia and Ukraine would negotiate directly with each other under Western auspices. The Russians reject the current leadership in Kiev as illegitimate. And although he said there were no plans for another takeover, Mr. Lavrov also repeated that the West should do more to curb the “lawlessness” in Ukraine; that formulation is often interpreted as a veiled warning that Russia might intervene if the West and its allies do not push the Ukrainian leadership to bring stability.

The move by Mr. Klitschko on Saturday could propel Mr. Poroshenko to a formidable lead in the election, where his most prominent anticipated contender is Ms. Tymoshenko, the country’s former prime minister and a familiar if controversial figure in the country’s fractious opposition movement. But it might also help Ms. Tymoshenko by removing one popular rival.

Mr. Klitschko said he would run instead for mayor of Kiev, with a goal of transforming the city into a “truly European capital.”

Mr. Poroshenko hailed the decision by Mr. Klitschko to step aside, saying it would serve the goals of the thousands of people who demonstrated for more than three months in hopes of putting Ukraine on the path to a pro-Western political future.

“It would be a betrayal if we did not unite,” Mr. Poroshenko said in a speech to the United Democratic Alliance for Reform congress Saturday.