Fierce fighting has been reported in Syria's second city of Aleppo, a day after the rebel Free Syrian Army (FSA) said a "decisive" battle had begun.
Both residents and activists described the clashes as "unprecedented", and said rebel fighters were attacking government positions on several fronts.
The fighting had spread to previously peaceful districts, they added.
On Thursday, the UN said the number of Syrians fleeing to other states could exceed 700,000 by the end of the year.
More than half a million are believed to have already crossed into Turkey, Jordan, Lebanon and Iraq, but only 294,000 have registered with the UN.
UN agencies and other humanitarian groups have issued a funding appeal for $488m (£300m) to help them meet the needs of the refugees.
Activists said the start of the rebel offensive in Aleppo was announced in calls from mosques at about 16:00 (13:00 GMT) on Thursday.
In interviews with foreign media and videos posted online, members of the Tawhid Brigade said a "decisive" battle for control of the city had begun. The offensive involved hundreds of rebel fighters attacking government positions on several fronts, they added.
"We wanted to surprise the Syrian army which had started to creep forward towards the southern neighbourhoods," Bashir al-Haji, the Tawhid Brigade's commander, told the Guardian newspaper.
He denied the FSA had declared "decisive" battles for Aleppo before.
"We are not aiming to liberate the whole of Aleppo with this battle but to regain control of most of the city and get back as many neighbourhoods as we can."
The rebel claims could not be immediately verified, but activists and residents reported heavy clashes and shelling in the districts of Izaa, Saif al-Dawla, Salah al-Din, Amariya and Sukkari on Thursday night and Friday morning.
"The fighting is unprecedented and has not stopped since Thursday," said Rami Abdul Rahman of the Syrian Observatory for Human Rights, a UK-based activist group, told AFP news agency.
But the BBC's Jim Muir in Beirut says there is little sign so far that the rebels have made much progress.
Syrian state media reported some unusual mortar fire onto south-eastern districts of the city which have so far been relatively untouched.
Attacks by rebels had been repulsed in several places, and heavy losses had been inflicted by government forces, they said.
For more than a month, the struggle for control of Aleppo has been at a stalemate, with government forces unable to dislodge the rebels, and the latter unable to take complete control, our correspondent adds.
Meanwhile, several north-eastern and southern districts of Damascus which were supposedly recaptured by the army weeks ago saw further violence on Friday, with troops and militia moving back in.
The Syrian Observatory for Human Rights said at least 60 people, including 30 civilians, had been killed across the country on Friday.