The United States and Russia are urging Syria's government and opposition groups to adhere to a revived cease-fire and allow humanitarian aid to flow to critical areas.

The two countries are the chief architects of the fragile truce. They're using a joint statement to show they're still committed to holding the truce together and resuming peace talks to end Syria's civil war.

The U.S. and Russia say they are also committed to developing a "shared understanding" of where the Islamic State and the al Qaida-linked Nusra Front hold territory. The groups are excluded from the cease-fire but are fighting alongside Western-backed rebels, complicating efforts to enforce the truce.

The statement comes as U.S. Secretary of State John Kerry attends a meeting in Paris of nations supporting the opposition.