Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina, who are among Obama's sharpest foreign policy critics and support greater U.S. assistance for Syria's rebels, said the agreement will embolden enemies such as Iran.
"What concerns us most is that our friends and enemies will take the same lessons from this agreement: They see it as an act of provocative weakness on America's part," they said in a joint statement. "We cannot imagine a worse signal to send to Iran as it continues its push for a nuclear weapon."
House Democratic leader Nancy Pelosi of California credited the president's "steadfast leadership" for "making significant progress in our efforts to prevent the use of weapons of mass destruction." She also credited Obama's "clear and credible" threats to use force against Syria for making the agreement possible.
U.N. inspectors were preparing to submit their report on the chemical weapons attack on the outskirts of Damascus on Aug. 21. Secretary-General Ban Ki-moon said Friday that he expected "an overwhelming report" that chemical weapons were indeed used.
A U.N. statement said Ban hoped the agreement will prevent further use of such weapons and "help pave the path for a political solution to stop the appalling suffering inflicted on the Syrian people."
Britain's foreign secretary, William Hague, said Saturday's development was "a significant step forward." Germany believes that "if deeds now follow the words, the chances of a political solution will rise significantly," Foreign Minister Guido Westerwelle said.
The commander of the Free Syrian Army rebel group, Gen. Salim Idris, said in Turkey that the Russian initiative would "buy time" and that rebels will continue "fighting the regime and work for bringing it down."
He said that if international inspectors come to Syria in order to inspect chemical weapons, "we will facilitate their passages but there will be no cease-fire." The FSA will not block the work of U.N. inspectors, he said, and the "inspectors will not be subjected to rebel fire when they are in regime-controlled areas."
Idris said Kerry told him by telephone that "the alternative of military strikes is still on the table."
Associated Press writer James Heintz in Moscow contributed to this report.