It also came after the Obama administration announced that it would retool a European missile defense system fiercely opposed by Russia. That move was thought to have paid dividends for the White House when Mr. Medvedev appeared to throw his support behind Mr. Obama on Iran, though American officials say the Russian president was also likely to have been reacting to the disclosure of the secret nuclear site near Qum.

After the meeting with Mr. Lavrov, Mrs. Clinton met Mr. Medvedev later on Tuesday, and two administration officials said he did not retreat from his support in his private discussions with her. But he said nothing about Iran publicly before or after the meeting. Prime Minister Vladimir V. Putin, who has been skeptical of sanctions, was in China on a trade mission.

Mr. Lavrov said that the talks in Geneva between Iran and other countries had raised hopes for a diplomatic solution, and that it made no sense to discuss sanctions as long as those negotiations were under way. “We are maybe not 100 percent, but still have chances to succeed,” he said.

His position conflicts with that of the Obama administration, which argues that the threat of sanctions is crucial to the prospect for a diplomatic solution. Unless Russia and China join the United States and Europe in signaling that the sanctions may be necessary if diplomacy fails, American officials have said, Iran has less incentive to make concessions.

A Russian refusal to back sanctions could expose the Obama administration to criticism at home, where Republicans have argued that the president yielded to Kremlin concerns on the missile shield without getting much in return.