Russia has told the United Nations it will stop bombing eastern Aleppo for 11 hours a day for four days, but that is not enough to trigger a wider ceasefire deal under which militants would leave the Syrian warzone, the UN said.
The Syrian military said a unilateral ceasefire backed by Russia had come into force to allow people to leave eastern Aleppo, a move rejected by rebels who say they are preparing a counter-offensive to break the blockade.
"They (the Russians) have said 11 hours per day and four days from today, Thursday," UN Syria humanitarian advisor Jan Egeland told reporters.
"We hope it can be four days from tomorrow Friday."
"They are considering that additional day," he said, noting that Russia had originally announced an eight-hour pause, but the UN had objected that it was too short to evacuate wounded and bring in assistance.
Kremlin spokesman Dmitry Peskov said a pause in Russian airstrikes on Syria's Aleppo may be extended, but not if rebels use the break to regroup or renew attacks.
Russia has said it stopped bombing because it expects fighters from the banned Jabhat Fateh al-Sham group, previously called the Nusra Front, to leave the city, under a ceasefire deal proposed by UN Syria envoy Staffan de Mistura.
But Mr de Mistura said he regarded the pauses in the bombing as a response to the UN request for medical evacuations.
His ceasefire proposal required more.
"The package is clear, Nusra needs to declare they are ready to go, or others can do on their behalf, and at the same time that there is a commitment by the (Syrian) government to respect the local administration," he said.
"Let's separate the two things. Today we consider this a medical evacuation, or medical support. The next steps are part of a larger package that needs to be there."
Mr Egeland said he hoped the first sick and wounded would be brought out on Friday to government-held western Aleppo or rebel-held Idlib, according to their choice.
"We hope to continue until we have been able to evacuate all of the cases that need such evacuation, and that would be hopefully done in the course of days, and it could be several hundred cases with their families."
UN aid convoys are ready to move from western Aleppo and from Turkey, but there was still no deal to get food into the besieged zone and no guarantee that all would go smoothly.
"This is Syria, so everything can go wrong at every possible opportunity," Mr Egeland said.
Mr de Mistura said there were "perhaps 6,000-7,000" rebel fighters in eastern Aleppo, apparently revising an estimate two weeks ago in which he said there were 8,000 rebels there, including up to 900 members of Jabhat Fateh al-Sham.
Clinton eyes negotiated no-fly zone
Meanwhile, US presidential frontrunner Hillary Clinton has stood behind her pledge to create a no-fly zone in Syria, but said it would have to be negotiated rather than imposed.
Acknowledging "legitimate concerns" about sparking a conflict with the Mr Assad's regime or its Russian backer - which has advanced weapons systems on the ground, Mrs Clinton said talks would be needed.
"I think a no-fly zone could save lives and hasten the end of the conflict," Mrs Clinton said in the final presidential debate of 2016.
"This would take a lot of negotiation and it would also take making it clear to the Syrians and Russians that our purpose is to provide safe zones on the ground."
