Residents in the eastern part of Aleppo in Syria have reported receiving SMS messages warning them to leave within 24 hours, or face a "strategic attack using high precision/advanced weapons".
They say the messages were sent through the Syrian state-controlled telecommunications provider MTN.
Rebels holed up in Aleppo can leave with their families if they lay down their arms, regime leader Bashar al Assad said on Thursday.
Fighters have accepted similar amnesty offers in other besieged areas in recent months, notably in Daraya, a suburb of Damascus that was under siege for years until rebels surrendered it in August.
More than 250,000 people are believed to be trapped inside eastern Aleppo, facing dire shortages of food and medicine. The Russian military along with regime forces have renewed their bombing campaign, following the breakdown of a US-brokered peace deal.
Speaking to Danish television, al Assad said he would "continue the fight with the rebels until they leave Aleppo. They have to. There's no other option."
However, rebels said they had no plan to evacuate Aleppo, the last major urban area they control, and denounced the amnesty offer as a deception.
"It's impossible for the rebel groups to leave Aleppo because this would be a trick by the regime," said Zakaria Malahifji, a Turkey-based official for the Fastaqim group which is present in Aleppo. "Aleppo is not like other areas, it's not possible for them to surrender."