“In a situation like Syria, I have to ask, can we make a difference in that situation?” Mr. Obama said. “Would a military intervention have an impact? How would it affect our ability to support troops who are still in Afghanistan? What would be the aftermath of our involvement on the ground? Could it trigger even worse violence or the use of chemical weapons? What offers the best prospect of a stable post-Assad regime?

“And how do I weigh tens of thousands who’ve been killed in Syria versus the tens of thousands who are currently being killed in the Congo? Those are not simple questions.”

More than 60,000 people have died in Syria’s nearly two-year-old conflict, but international efforts to end the crisis appear stalled. The opposition is divided, and Russia, the main backer of Syria’s president, Bashar al-Assad, is at loggerheads with the Syrian opposition’s Western and Arab supporters.

Russia’s prime minister, Dmitri A. Medvedev, told CNN that Mr. Assad’s chances of remaining in office seemed to be getting “smaller and smaller” with each passing day, according to a transcript released by Mr. Medvedev’s office on Sunday. But he reiterated Russia’s insistence that Mr. Assad’s ouster could not be a precondition for talks, as the American-backed Syrian opposition leaders have demanded.

Mr. Medvedev said the United States, Europe and regional powers must “sit the parties down for negotiations, and not just demand that Assad go and then be executed” like Libya’s ousted leader, Col. Muammar el-Qaddafi, “or be carried to court sessions on a stretcher like Hosni Mubarak,” the deposed Egyptian president.

If Mr. Assad is to step down, “this must be decided by the Syrian people,” he said, “not Russia, not the United States, not any other country.”

Fighting edged into a new area of Damascus, the capital, according to activists, who said rebels attacked a railway station in the district of Qadam, in the city’s southwest. Video posted on the Internet showed gunmen walking near buildings by a railroad track and black smoke that activists said was from an airstrike. The claims were impossible to verify because of the government’s restrictions on journalists inside Syria.