Damascus has agreed to halt its aerial bombardment of Aleppo for six weeks for a trial ceasefire, UN envoy to Syria Staffan de Mistura says.
Staffan de Mistura said the Syrian pledge offered a glimmer of hope although it is unclear when it would take effect.
The opposition Syrian National Council says the government will be judged by actions rather than words.
Government forces have been engaged in heavy fighting as they try to cut a crucial rebel supply route.
More than 100 soldiers and rebels were reported to have been killed on February 17 as the army captured several villages north of the city.
Staffan de Mistura has been working since October to negotiate what are called “local freezes” in the Syrian fighting that would start with Aleppo.
During a recent visit to the country, he held a long meeting with Syrian leader Bashar al-Assad, during which the Syrian president apparently indicated a willingness to halt all aerial bombing and artillery shelling of Aleppo for the six-week period.
Staffan de Mistura said he would return to Syria as soon as possible with the aim of announcing a freeze in the fighting in the northern city, during which opposition forces would also be expected to suspend their mortar and rocket fire.
“Let’s be frank, I have no illusions because based on past experiences, this will be a difficult issue to be achieved,” he said.
Aleppo has been divided between rebel and government control since fighting erupted in the city in mid-2012.
Soldiers, backed by pro-government militiamen and fighters from Lebanon’s Hezbollah movement, have made steady advances since launching an offensive to retake the rebel-held west of the city last year.
Aleppo-based rebels and opposition activists have expressed concerns that the government will exploit any local truces to redeploy its forces to fight elsewhere, and have questioned how they will work with jihadist militants from the Islamic State (ISIS) group in the area.