Russia President Vladimir Putin said on Thursday that Moscow was ready to extend the humanitarian pause in Aleppo, Syria's second largest city already badly damaged by bombing, "as far as possible".

"We have made clear our intention to extend as far as possible, depending on the current situation on the ground, the halt in our air strikes," Putin said during a press conference broadcast on Russian television following negotiations on Syria with French leader Francois Hollande and German Chancellor Angela Merkel in Berlin.

The Syrian Army said on Wednesday the truce would extend to three days, the state news agency SANA reported. Initially, Russia said the humanitarian halt in fighting would last 11 hours.

The truce in rebel-held eastern districts of Aleppo city will take place on "20, 21 and 22 October, from 8:00 am (0500 GMT) till 16:00 pm (1300 GMT)," the army announced in a statement late Wednesday, according to SANA.

The army called on "the armed men in the eastern districts of Aleppo to leave their weapons".

However, ground fighting continued in the historic Old City, a monitor said.

Washington had voiced scepticism about how long the lull in anticipation of a wider ceasefire would last.

But 24 hours after the start, the Syrian Observatory for Human Rights said there had been no air strikes on rebel-held east Aleppo, which had been heavily bombed since the army launched an offensive to recapture it on 22 September.

"There have been no air raids from yesterday morning until now," Observatory head Rami Abdel Rahman told AFP.

Moscow said the lull was intended to give the more than 250,000 civilians trapped in east Aleppo time to prepare to leave the city.

Government forces, which have kept rebel areas under near-continuous siege since mid-July, have said they will open six corridors for the safe passage of fleeing civilians.

The ceasefire is scheduled to begin at 5am GMT on Thursday.

On Wednesday, troops pressed their ground assault in the Old City as they vied to push back the front line in the heart of Aleppo that has remained largely static since the rebels seized the east in 2012.

Ibrahim Abu al-Leith, a spokesman for the White Helmets rescue force in Aleppo, said there were no planes circling above but artillery and rocket fire continued.

"It's better than before, but people won't go out unless everything stops. They are still scared because they know that the regime and Russia are not trustworthy," Abu al-Leith told AFP.

The United Nations has welcomed Russia's announcement, but has said that it is waiting for safety assurances from all sides before entering Aleppo to deliver relief supplies and evacuate wounded civilians.