BEIRUT - The diplomatic push to end the fighting in Syria gained new momentum Thursday as a top U.N. official endorsed the formation of a transitional government and Russia signaled that it might be prepared to pressure President Bashar Assad's regime to negotiate.

The outlines of any possible breakthrough remain vague, but recent statements by diplomats in Beirut, in Moscow and at the United Nations underline a sense that substantive negotiations could still take place. At the same time, there are emerging fears that the prospects for a negotiated settlement may slip away, as high-level defections from the Syrian government and rebel military successes embolden the opposition to fight to victory.

On Thursday, Lakhdar Brahimi, the joint U.N.-Arab League envoy to Syria, said that a plan discussed during an international conference on Syria in Geneva in June could be used as a blueprint to establish an interim government with "full executive powers" ahead of elections. He also said, however, that the Syrian government structures must be kept intact, a requirement unacceptable to many in the opposition, who want guarantees that Assad and other senior regime officials will be barred from holding office.

"This transitional period should not be allowed to lead to the collapse of the state and its institutions," Brahimi said at a news conference in Damascus, the Syrian capital.

Brahimi's comments came as Russian Foreign Minister Sergei Lavrov spoke out in favor of a political solution to Syria's 21-month-old conflict.