The UN on Wednesday dashed hopes that a prospective ceasefire in east Aleppo could provide humanitarian relief for besieged civilians, saying all sides needed to back an extended truce before aid convoys could roll.
A plan to pause fighting for eight hours in rebel-held east Aleppo from 0500 GMT on Thursday has been announced by Assad regime ally Russia -- with no word from the Damascus government that it intends to honour the deal.
"Before we can do something really meaningful... we need assurances from all parties," said Jens Laerke, spokesman for the United Nations humanitarian agency (OCHA) in Geneva.
"We need a minimum of 48 hours", he added, stressing that Russia's time-limited deal would not be sufficient for UN teams to mobilise.
Aside from getting desperately needed supplies to east Aleppo's estimated 250,000 civilians, the UN also wants to evacuate hundreds of people in urgent need of medical care.
UN and Red Cross trucks packed with aid for east Aleppo have been stalled near the Turkish border for weeks, waiting for guarantees that they can safely make deliveries.
Moscow has said Thursday's proposed truce could give civilians an opportunity to flee the city.
There has been a pause in air strikes over east Aleppo since Tuesday, a lull Russia says could allow residents time to prepare to leave.
Syrian and Russian jets have been bombarding the area in a brutal offensive to retake the key city from rebel fighters.
While Washington has voiced scepticism over the prospects for Thurday's truce, French President Francois Hollande and German Chancellor Angela Merkel were due to meet with Russian leader Vladimir Putin in Berlin later on Wednesday to discuss the plans.