At least 12 people have been killed in rebel rocket and gunfire in western parts of the divided Syrian city of Aleppo, state media report.
State media blamed the attack on rebels based in the east of the city. At least 200 people were injured.
Russia and Syria have told the rebel forces to leave eastern Aleppo by Friday evening but the rebels have rejected the offer.
Government forces have been besieging the rebel-held east since July.
Russia is supporting the government troops with air strikes. More than 250,000 trapped residents have been enduring food shortages, price hikes and relentless bombing.
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 Syrian Observatory for Human Rights said.
The UK-based monitoring group told the AFP news agency that the “most intense fighting” was on the fringes of the Halab al-Jadida area where it is believed the rebels are concentrating their efforts to break the siege.
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 would be opened for civilians, the ministry said.
Previous attempts at humanitarian exit pauses have largely failed as the government and the rebels accuse each other of stopping people leaving.
In a statement, the Syrian army said it called on “all gunmen to stop hostilities and take the opportunity of the humanitarian grace to leave the city with their light weapons through the Castello road to the north or the Souq al-Khair – Masharqa towards Idlib”.
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The corridors would be open between 09:00 and 19:00 local time (06:00 and 16:00 GMT) on Friday, the army said.
But the rebels have rejected the offer, emphasising they will not surrender and will continue their efforts to break the siege.
Syria conflict: Attack on government-held Aleppo ‘kills 12’}