Russian and Syrian government forces have begun a 10-hour ceasefire in the city of Aleppo to allow rebels and civilians to leave besieged areas.
Russia has said this pause will be the last chance for rebels to leave eastern districts in safety.
The rebels have rejected the offer and have been continuing a counter-offensive aimed at breaking the siege.
About 250,000 people remain trapped and are enduring food and medical shortages as well as intensive bombing.
A BBC correspondent in Aleppo says the battle in the city is escalating, with the government determined to recapture districts still outside their control while the rebel forces do not want to lose their last remaining foothold in a major Syrian city.
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The so-called “humanitarian pause” is the second time Russia and the Syrian government have said passages are open for evacuations, but their last unilateral ceasefire in October ended with almost no-one leaving over three days.
Rebels can leave unharmed and with their weapons using two specially created corridors between 09:00 and 19:00 (06:00 and 16:00 GMT), the Russian defence ministry said. Six other routes were to be opened for civilians.
Russian and Syrian warplanes are expected to resume attacks on rebel areas when the truce ends.
On Thursday rebels in east Aleppo increased their attacks on western government-held areas of the city.
State media said at least 12 people had died and 200 were injured in rocket fire, gunfire and car bombs.
Government forces have been besieging the rebel-held east almost continually since July.
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.