Russia says talks are taking place to include Aleppo in a temporary lull in fighting declared by the Syrian army in some western parts of the country, a sign of intensified efforts to halt a surge of violence in Syria's former commercial capital.
The United States said stopping the bloodshed in Aleppo, which has been at the centre of an escalation of violence that has all but destroyed a wider ceasefire deal and broke up peace talks in Geneva, was a top priority.
Nearly 10 days of bombardments by both the government side and insurgents in the city of Aleppo has killed more than 250 people, a monitoring group said, confounding hopes of an end to five years of war.
Rebels shelled at least one area of the city on Sunday (local time) and government warplanes raided its outskirts, but bombardments were fewer, the Syrian Observatory for Human Rights said.
Moscow and Washington brokered the February 27 ceasefire deal, which applied to western Syria but excluded al-Qaida and Islamic State fighters. World powers and the United Nations have been trying to salvage that truce.
Syria's army announced late on Friday a "regime of calm", or lull in fighting, which applied to Damascus and some of its outskirts, and parts of northwestern coastal province Latakia.
But it excluded Aleppo.
US Secretary of State John Kerry, who arrived in Geneva on Sunday for discussions on Syria with the United Nations and his Jordanian and Saudi counterparts, said a ceasefire was needed throughout Syria and hoped to be able to reaffirm the cessation of hostilities after talks in Geneva.
A senior defence ministry official in Moscow, which supports Syrian President Bashar al-Assad, said on Sunday negotiations were taking place to "establish a regime of calm also in Aleppo province", Interfax news agency reported.
He said the lull in fighting had been extended around Damascus for another 24 hours. In Latakia province, it still applied through Monday without need for an extension. It had been respected in both areas, the Russian official said.
Syria's army confirmed the extension of the lull around Damascus but did not mention Aleppo.
A number of rebel groups late on Saturday rejected the partial "regime of calm" in Damascus and Latakia, saying any truce must include all areas where the government and main opposition were fighting, as under the February deal.
"We will not accept under any circumstances... regional ceasefires," they said, adding they would respond as "one bloc" to attacks in any area of the country.
The US State Department said Washington wanted Russia to put pressure on Assad to stop what it called indiscriminate aerial attacks in Aleppo, Syria's biggest city before the war, and which has long been split between government- and opposition-held areas of control.
"These are critical hours," Kerry said, adding: "We look for Russia's cooperation, and we obviously look for the regime to listen to Russia and to respond."