The United States said on Wednesday it had agreed with Russia to extend a cessation of hostilities agreement to include Aleppo where intense day-long violence between Syrian rebels and government forces killed dozens of people, Reuters reported.
The State Department said the truce went into effect at 12:01 a.m. Damascus time on Wednesday, but acknowledged the fighting had not stopped.
U.S. Secretary of State John Kerry said he was not surprised that fighting continued in some areas, adding both sides were working to communicate with commanders in the field.
Kerry, meeting with EU foreign policy chief Federica Mogherini at the State Department, said it was vital that both sides abide by the agreement. He called on Russia to use its influence over President Bashar al-Assad to stop the violence.
There was no immediate response from Moscow to the announcement of an agreement, but the Syrian army said it would implement a "regime of calm" in Aleppo for 48 hours as of Thursday.
The surge in bloodshed in Aleppo, Syria's largest city before the civil war and biggest strategic prize, wrecked the first major "cessation of hostilities" agreement of the war, sponsored by Washington and Moscow, which had held since February.
In battles on Wednesday between rebels and government forces in western Aleppo, opposition forces said they were forced to retreat by heavy aerial bombing.
Russia's U.N. ambassador, Vitaly Churkin, addressing a U.N. Security Council meeting on the situation in Aleppo, said an agreement would have been announced on Tuesday but opposition attacks in Aleppo had prevented it from happening.
"The deterioration in certain areas of Syria, including Aleppo, is a serious source of concern. The government forces are fighting off a large-scale offensive by the jihadists (in Aleppo)," he told the council.