GENEVA -- The UN hailed an announcement Tuesday that Russian and Syrian air forces have stopped bombing Aleppo, but said it was waiting for safety assurances from all sides before going in with aid.

"We remain of course pleased if there is a reduction in fighting...because it will protect civilians," Jens Laerke, spokesman for the UN humanitarian agency, told reporters.

But he stressed that "when the weapons fall silent, we need all weapons to fall silent," before "critical humanitarian assistance can be provided to the area and the sick and the wounded can be evacuated."

"We need to have assurances from all parties to the conflict. Not just a unilateral announcement that this will happen," he said, without revealing if the UN had received security assurances from any of the sides in the conflict.

Laerke's comments came after the Russian defence minister Sergei Shoigu announced that Russian and Syrian air forces had stopped bombing Aleppo as of 7 a.m. GMT, in a move he said was meant to pave the way for an eight-hour truce on Thursday.

Shoigu said the "humanitarian pause" on Thursday would allow civilians to exit through six corridors and would make it possible to evacuate the sick and wounded from eastern Aleppo.

The UN and European Union welcomed the announcement, but said the planned pause in fighting needed to be longer to allow the delivery of humanitarian aid.

"We continue to call for a 48-hour pause," Laerke said.

"The UN has a plan to evacuate the sick and wounded from eastern Aleppo, but has not been involved with any negotiations" around the Russian announcement, he said.

"Movement of civilians must be done on a safe, dignified and voluntary basis, and should not be mixed with the UN plan to evacuate the sick and wounded," he said.

Once Syria's thriving commercial hub, Aleppo has been ravaged by air raids and intensifying clashes as regime forces fight to capture the rebel-held east.

Russian air support for the onslaught -- which has destroyed hospitals and other civilian infrastructure -- has spurred accusations of potential war crimes.

More than 300,000 people have been killed since Syria's war devolved from a widespread protest movement against Assad's rule in March 2011 to a multi-front war between rebels, jihadists, Kurds and regime forces. (AFP)