The US says it has 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.
The State Department said the truce went into effect at 12.01am Damascus time on Wednesday, but acknowledged the fighting had not stopped.
US 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 under heavy aerial bombing.
Russia's UN ambassador Vitaly Churkin, addressing a UN 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.
Kerry said the US was co-ordinating closely with Russia to finalise strengthened monitoring of the extension of the cessation of hostilities to Aleppo.
He said he expected a meeting of the International Syrian Support Group, a grouping of foreign ministers of European and Middle Eastern government chaired by Kerry and Russian Foreign Minister Sergei Lavrov, to meet within the next two weeks.
In Berlin, the German and French foreign ministers said achieving a ceasefire in Aleppo was critical to renewing peace talks.
"I believe everyone knows and can conclude that there could be no return to the political talks in Geneva if a ceasefire in and around Aleppo is not observed," German Foreign Minister Frank-Walter Steinmeier told reporters.
In Geneva, a senior UN humanitarian official said the Syrian government was refusing UN demands to deliver aid to hundreds of thousands of civilians trapped by the fighting, including many in devastated Aleppo.
The Syrian Observatory for Human Rights said dozens had been killed on both sides in what it described as the most intense battle in the Aleppo region in a year. Government forces were reinforced by allies from Lebanon's Hezbollah, it said.
"The current levels of violence in Aleppo, in particular, negatively impact the ability of the Syrian parties to engage in negotiations," UN political chief Jeffrey Feltman said during the Security Council meeting.
UN aid chief Stephen O'Brien told the 15-member council that life for the people in Aleppo was horrendous and they were "living under daily threat and terror".
