WASHINGTON — A shaky pause in major fighting went into force in Syria at sundown Sept. 12, three days after the United States and Russia announced they had reached a deal to try to establish a nationwide cease-fire, expand access to humanitarian aid and make way for Syrian combatants to return to the negotiating table.

US Secretary of State John Kerry urged all the parties to the cessation of hostilities — including a skeptical Syrian opposition — to support the truce deal that would eventually see the United States and Russia coordinating on the targeting of al-Qaeda’s Syria affiliate, Jabhat Fatah al-Sham (formerly known as Jabhat al-Nusra). Kerry warned its failure could lead to an escalation of the conflict, which would make keeping Syria a unitary nation difficult after five brutal years of civil war that have killed an estimated half a million people.

 “I believe this is the only realistic solution to this conflict,” Kerry told journalists at the State Department on Sept. 12. “I urge all parties to support it. It may be the last chance one has to save a united Syria.”

Under the plan, announced by Kerry and Russian Foreign Minister Sergey Lavrov after daylong talks in Geneva on Sept. 9, a seven-day period of calm or at least reduced violence starting Sept. 12 would then lead to the establishment of a US-Russian Joint Implementation Center (JIC), which would share intelligence and coordinate the targeting of Jabhat Fatah al-Sham as well as the so-called Islamic State (IS).