Rebel fighters in east Aleppo have intensified their attempts to break the Syrian government's siege, ahead of what Russia says will be their last chance to leave the city in safety.
Many people have been killed or injured in rebel attacks on western, government-held areas.
Russia, a key ally of the Syrian government, says rebel fighters may leave with their weapons during a 10-hour window on Friday.
But rebels have rejected the offer.
They said they would not surrender and would continue their efforts to break the siege.
After the brief lull in fighting, Russian and Syrian warplanes are expected to resume bombing rebel areas, where about 250,000 people remain trapped and are enduring food shortages, price hikes and scant medical capacity as well as the bombing.
Government forces have been besieging the rebel-held east since July.
State media said on Thursday that at least 12 people had died in rocket fire, gunfire and car bombs in government-held western Aleppo.
They blamed the attack on rebels in the east of the city, and said at least 200 people were injured.
The BBC's Owen Bennett-Jones in western Aleppo says that mortar fire both by rebels and pro-government forces has been in the air throughout Thursday and that parts of the west of the city - like many parts of the east - have been reduced to rubble.
In fighting on Thursday the rebels attacked government forces in several districts of Aleppo's western outskirts, the UK-based Syrian Observatory for Human Rights said.
Meanwhile 300km (185 miles) to the north, a Russian helicopter was damaged by mortar fire while delivering aid, near Palmyra in Homs province, the country's military said.
It could not fly back to base but the crew were rescued and are safe.
The so-called Islamic State group may be behind the attack. It claimed to have shot down a Russian attack helicopter in Homs province in a strike which it said killed the crew.
Russian President Vladimir Putin offered a new unilateral pause in fighting in Aleppo on Wednesday, urging rebels to use the time to gather their things and leave.
Russia says that the president wants to avoid senseless bloodshed. Both Russia and Syria say they have not bombed eastern Aleppo for more than two weeks.
The Russian defence ministry says rebels can leave unharmed and with their weapons using two specially created corridors. Six other routes will be opened for civilians, the ministry says.
Previous attempts at humanitarian exit pauses have largely failed as the government and the rebels accuse each other of stopping people leaving.
The corridors would be open between 09:00 and 19:00 local time (06:00 and 16:00 GMT) on Friday, the army said.