Russia has been the chief international defender of Assad’s regime, a military and trade partner, and the main obstacle to tougher U.N. action to pressure him to end the war and step aside. No decisions emerged from Thursday’s three-way discussion, but a State Department official said the talks had been constructive. Lakhdar Brahimi, the U.N.-Arab League envoy to Syria, said the group was seeking a “creative” solution to the Syria crisis.

The meeting came amid fresh concerns that a desperate Assad might resort to using chemical weapons against the rebels or civilians. Clinton would not directly address reports that Assad’s army has prepared deadly sarin gas for delivery by missile.

“Events on the ground in Syria are accelerating, and we see that in many different ways,” she said ahead of the meeting. “The pressure against the regime in and around Damascus seems to be increasing. We’ve made it clear what our position is with respect to chemical weapons.” President Obama has warned the Syrian government that the use of such munitions would cross a “red line” and trigger U.S. intervention.

Defense Secretary Leon E. Panetta also was asked whether the threat appears imminent. Panetta did not say what new intelligence information he has about Assad’s intentions, but he said at a news conference in Washington that it is enough to raise U.S. fears.

“I think there is no question that we remain very concerned, very concerned that as the opposition advances, in particular on Damascus, that the regime might very well consider the use of chemical weapons,” Panetta said. “The intelligence that we have causes serious concerns that this is being considered.”

Other U.S. officials said this week that intelligence agencies have detected that Assad’s government has been preparing its chemical weapons stockpiles for possible use. About 40,000 people have died in the uprising.

On Wednesday, Clinton called Assad’s fall “inevitable,” but Russian officials have long believed that he can defeat the rebels.

“We have been trying hard to work with Russia to stop the bloodshed in Syria and start a political transition towards a post-Assad Syrian future,” Clinton said at a news conference before the meeting.

The session among Clinton, Russian Foreign Minister Sergei Lavrov and Brahimi came at the U.N. envoy’s invitation. He and other would-be peacemakers say that a lasting solution would require agreement between the United States and Russia. The United States is the largest player in deciding any international response or intervention in Syria, and Russia is Syria’s patron.