“Secretary Clinton has accepted an invitation by U.N. Special Envoy Brahimi for a trilateral meeting on Syria this afternoon with Mr. Brahimi and Russian Foreign Minister Lavrov,” a senior State Department official said Thursday morning.

This is not the first time that American and Russian consultations have spurred hopes of a possible breakthrough. In June, Mrs. Clinton, Mr. Lavrov and the United Nations’s envoy on the Syrian crisis at the time, former United Nations Secretary General Kofi Annan, appeared to be close to an agreement that a transitional government should be established and that President Bashar al-Assad give up power.

But that seeming understanding quickly broke down, with Americans officials complaining privately that the Russian side had pulled back from the deal. A major sticking point, it later emerged, was the American insistence that the United Nations Security Council authorize steps to pressure Mr. Assad if he refused to go along under Chapter 7 of the United Nations Charter, which could be used to authorize tougher economic sanctions and, in theory, the use of force.