WASHINGTON — The United States and Russia announced an agreement on Monday for a partial truce in Syria, though the caveats and cautious words on all sides underscored the obstacles in the way of the latest diplomatic effort to end the five-year-old civil war.

Under the terms of the agreement, the Syrian government and Syria’s armed opposition are being asked to agree to a “cessation of hostilities,” effective this Saturday. But the truce does not apply to two of the most lethal extremist groups, the Islamic State and the Nusra Front, raising questions about whether it will be any more lasting than previous cease-fires.

The agreement calls for the Syrian government and the opposition to indicate by noon on Friday whether they will comply with the cessation of hostilities, a term carefully chosen because it does not require the kind of agreement in a formal cease-fire. The United States is responsible for bringing the various opposition groups in line while the Russians are supposed to pressure the government. Washington and Moscow also agreed to set up a hotline to monitor compliance by both sides.

President Obama sealed the final terms of the arrangement in a phone call with President Vladimir V. Putin of Russia, who has become perhaps the most influential player in the Syrian war since Russia thrust itself into the conflict in September on behalf of its client, Syria’s president, Bashar al-Assad.