The US State Department voiced skepticism Tuesday over a temporary ceasefire called by Russia in Aleppo, although Washington welcomed the break in the bombardment of the war-scarred Syrian city.

"Certainly we're gratified to hear the reports that there might have been a reduction here in the violence," State Department spokesman John Kirby told CNN, a day after Moscow announced the truce.

"It's a little too soon to tell how genuine this is and how long it's going to last. We've seen these kinds of commitments and promises before. And we've seen them broken. We're watching this very carefully," Kirby said.

"Now we have to see if they can actually put the muscle behind it."

Russia announced an eight-hour ceasefire in Aleppo commencing at 0500 GMT on Thursday to allow time for Syrian civilians to flee the besieged city and to permit humanitarian workers to deliver aid.

But they halted air strikes on Tuesday, in what the Kremlin said was a "goodwill" gesture, amid mounting criticism of Russia for backing the brutal regime of Syrian President Bashar al-Assad.

The United Nations repeatedly has called for 48-hour weekly pauses in the bombing to allow aid convoys to reach besieged civilians living in Aleppo.

Syria's second city, held by rebels determined to oust Assad, has come under heavy bombardment since the Russia-backed military announced an offensive in late September to regain control of the east.

Air strikes in the city have flattened numerous residential buildings and civilian facilities, in a bombing campaign the European Union warned could amount to war crimes.