GENEVA — The United States and Russia reached a sweeping agreement on Saturday that called for Syria’s arsenal of chemical weapons to be removed or destroyed by the middle of 2014 and indefinitely stalled the prospect of American airstrikes.

The joint announcement, on the third day of intensive talks in Geneva, also set the stage for one of the most challenging undertakings in the history of arms control.

“This situation has no precedent,” said Amy E. Smithson, an expert on chemical weapons at the James Martin Center for Nonproliferation Studies. “They are cramming what would probably be five or six years’ worth of work into a period of several months, and they are undertaking this in an extremely difficult security environment due to the ongoing civil war.”

Although the agreement explicitly includes the United Nations Security Council for the first time in determining possible international action in Syria, Russia has maintained its opposition to any military action.