Russia’s top diplomat held a hurried private discussion Thursday with Secretary of State Hillary Rodham Clinton and the U.N. envoy for Syria about the 20-month-old civil war in the country that is Russia’s closest Middle East ally.

The meeting is a sign that Russia may be reconsidering support for Syrian President Bashar al-Assad, purely as a practical response to his weakening military position. Until now, Russia has rejected U.S. and other calls to abandon Assad and has appeared to think that he can defeat the rebels and keep his government intact.

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.

After the 40-minute session, Brahimi said that no major decisions were taken but that the three parties had agreed to work together.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

“We have agreed that the situation is bad, and we have agreed that we must continue to work together to see how we can find creative ways of bringing this problem under control.”

The State Department official, who insisted on anonymity, said in a written statement that the meeting had focused “on how to support a political transition in practical terms’’ and that American and Russian officials would meet again with Brahimi within days “to discuss the specifics of taking this work forward.’’

The U.S.-Russian discussion took place on the sidelines of an unrelated meeting of the ­election-monitoring Organization for Security and Cooperation in Europe.

It came ahead of a gathering of the Western-backed Friends of Syria group in Morocco next week, at which the United States is expected to recognize a reorganized Syrian political opposition as the legitimate successor to the Assad regime.

Russia is highly unlikely to back any military action against the Syrian government, and U.S. officials say military action is not their goal. It was also not clear that Russia intends to withdraw its support for Assad, but Lavrov’s willingness to attend the meeting indicated that the Kremlin is exploring its options.

Brahimi took over the envoy role from Kofi Annan, whose plan for a cease-fire and peace talks for Syria fell apart earlier this year.

Brahimi has called for a U.N. Security Council resolution based on a deal that world powers reached in Geneva in June to set up a transitional Syrian government in a bid to end the conflict. That document did not specify what role, if any, Assad would play in such a transitional administration.

U.S. officials say that a stronger U.N. mandate would put further pressure on Assad to step down and that Russian agreement over such an effort would leave the Syrian president with no meaningful international support. Any withdrawal of Russian backing for Assad could help convince him that he cannot hold on to power any longer, the officials say.

Russian intervention is likely only if President Vladi­mir Putin concludes that the Syrian government will be defeated. Putin wants guarantees that Russia will keep its naval base and preferential trade ties even in a post-Assad Syria; experts say that one way of maintaining that influence could be for Moscow to join international efforts to push and plan for Assad’s departure.

Syria’s deputy foreign minister said Thursday that Western powers were whipping up fears about the use of chemical weapons in Syria’s civil war as a “pretext for intervention.”

Meanwhile, Obama administration officials say they are continuing to encourage those close to Assad to defect.