Opposition fighters denounce temporary ceasefire as effort by Russia and Syrian government to seize rebel-held areas.
The Syrian military said on Thursday a unilateral ceasefire backed by Russia had come into force to allow people to leave besieged eastern Aleppo, a move rejected by rebels who say they are preparing a counter-offensive to break the blockade.
Rebels say the goal of Moscow and Syrian President Bashar al-Assad is to empty opposition-held areas of civilians so they can take over the whole city.
"They talk about humanitarian corridors, but why are they not allowing food into besieged eastern Aleppo to alleviate our suffering? We only need the Russian bombers to stop killing our children. We don’t want to leave," said Ammar al-Qaran, a resident in Sakhour district.
Syrian state-owned Ikhbariyah television said rebels had fired a mortar barrage near to where ambulances had been heading to take patients from the besieged parts of the city for treatment in government-held areas.
Also on Thursday, a UN aid official for Syria said Russia agreed to extend daily pauses in military action against rebel-held eastern Aleppo for four more days.
Jan Egeland told The Associated Press news agency that the UN on Thursday received verbal assurances for the extension by a day - from three days previously - both from Russia’s diplomatic mission in Geneva and in writing from Russian military officials in Syria.
Egeland noted the UN had already received assurances a day earlier from Moscow that the daily pauses in air strikes and artillery shelling would be extended from eight hours to 11 hours per day.
"We have gotten it extended in both hours and in terms of days," he said, adding the UN has "a window from Friday at least until Monday".
Egeland said the UN received "green lights" it needed from Syria’s government, armed opposition groups, and Russia, which announced the pause in fighting that began on Thursday.
He said the UN hoped to organise evacuations of "several hundred" critically wounded or sick people with their families, either to government-controlled western Aleppo or to the rebel-held city of Idlib to the southwest, and deliver medical supplies to eastern Aleppo.
An estimated 275,000 civilians remain trapped in the city’s east.
Russia’s Defence Minister Sergei Shoigu said on Thursday the "humanitarian pause" has been extended on President Vladimir Putin’s instructions and was supported by the Syrian government.
Putin said Russia was ready to extend the pause in air strikes if the rebels did not escalate fighting. The Kremlin specified that the rebels’ attempt to re-arm and re-group would derail the humanitarian pause.
Meanwhile, German Chancellor Angela Merkel pressed anew for a long-term ceasefire in Aleppo.
Merkel said as she arrived on Thursday at a European Union summit in Brussels that she hoped EU leaders would "make clear that what is happening in Aleppo, with Russian support, is completely inhuman".
"There must be work as soon as possible on achieving a ceasefire - not just one over several hours per day, followed by many hours of bombing, but a lasting ceasefire."
Since Russia intervened in the war more than a year ago, the government’s side has gained the upper hand on numerous fronts, including Aleppo, where the opposition-held sector has been completely encircled for weeks.
The Syrian army has pressed ahead with a major campaign, supported by Iranian-backed militias and Russian air power, to take full control of Syria’s largest city, divided between rebel and government zones since 2012.
The rebels, however, said they were preparing a large-scale offensive to break the siege of Aleppo.
"The coming battle is not going to be like others. We are waiting for the signal of the start of a decisive battle which will surprise the regime and its militias," Abu Obeida al-Ansari, a commander from Jabhat Fateh al-Sham, formerly known as al-Nusra Front, said in a statement on social media.