A pause in Russian and Syrian government air strikes on rebel-held areas of Aleppo held for a second day Wednesday, although ground fighting continued in the historic Old City, a monitor said.

Washington had voiced scepticism about how long the lull announced by Moscow from 0700 GMT on Tuesday 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 September 22.

"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 during the eight-hour ceasefire announced for Thursday.

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 0500 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.

French President Francois Hollande and German Chancellor Angela Merkel are to meet with Russian leader Vladimir Putin in Berlin later on Wednesday to discuss the ceasefire plans.

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.

A September 19 air strike, which Western governments have blamed on Moscow, hit a 31-truck convoy delivering aid to a town west of Aleppo.

Washington has expressed scepticism about Moscow's intentions 13 months after it launched its air war in support of President Bashar al-Assad.

"It's a little too soon to tell how genuine this is and how long it's going to last," State Department spokesman John Kirby told CNN.

"We've seen these kinds of commitments and promises before. And we've seen them broken. We're watching this very carefully."