Heavy clashes erupted between regime and rebel forces in Syria's divided city of Aleppo after a unilateral ceasefire announced by Russia expired, according to the Syrian Observatory for Human Rights (SOHR).
The first Syrian or Russian air strikes on Aleppo, Syria's most populous city before the civil war, since Russia began the pause in hostilities on Thursday hit a key frontline in the city's southwest.
Ground clashes and shelling, which had continued throughout the day on frontlines, intensified.
The pause began on Thursday, and came after Russia announced a temporary halt to the Syrian army's campaign to recapture the divided city.
Moscow had extended the unilateral "humanitarian pause" into a third day until 1600 GMT Saturday, but announced no further renewal of the truce despite a UN request for longer to evacuate wounded civilians.
"Members of popular civil committees from regime districts entered the eastern neighbourhoods to try to evacuate the injured but failed," SOHR head Rami Abdel Rahman said.
The SOHR reported fierce fighting in several areas of Aleppo, with three people wounded by shelling of the rebel-held Salaheddin and Al-Mashhad districts.
No civilians were evacuated during the truce, Ingy Sedky, spokeswoman for the International Committee of the Red Cross in Syria, said, adding that evacuations were impossible due to the poor security situation in the city.
Syrian state media and Russian authorities have accused rebels in the east of preventing civilians from leaving and using them as "human shields".
More than 2,000 civilians have been wounded since the army launched its offensive to drive the rebels out of the eastern districts they have held since 2012. Nearly 500 people have been killed.
In Pictures: Syrian refugees - Displaced by war
About 250,000 to 300,000 civilians are thought to be trapped in eastern Aleppo, with dwindling food supplies and extremely limited medical care in underground hospitals that have themselves been hit repeatedly by air strikes.
The UN had hoped to use the ceasefire to evacuate seriously wounded people, and possibly deliver aid. But a UN official said the requisite security guarantees had not been received.
"You have various parties to the conflict and those with influence. They all have to be on the same page on this and they are not," said David Swanson, a spokesman for the UN humanitarian office.
No aid has entered Aleppo since July 7 and food rations will run out by the end of the month, UN chief Ban Ki-moon warned on Thursday.
The UN had asked Russia to consider extending the pause until Monday evening, but there was no word of any extension as the 1600 GMT deadline passed.
Moscow accuses rebels of preventing civilians from leaving, with senior Russian military official Sergei Rudskoi accusing them of "using the ceasefire in their interests".
Russia is a key ally of Syria's government and began a military intervention in support of President Bashar al-Assad last September.
Elsewhere in Aleppo province, Turkish forces and allied Syrian rebels were shelling the Kurdish-Arab Syrian Democratic Forces coalition in several villages.
On Friday, a UN human rights council resolution called for "a comprehensive, independent special inquiry into the events in Aleppo".
It also demanded that warring parties provide unrestricted humanitarian access to desperate civilians and "end immediately all bombardments and military flights over Aleppo city".
Also Friday, UN experts said the Syrian army was responsiblefor a March 2015 chemical weapons attack on the village of Qmenas.