Syrian rebels said they had launched a major attack in Aleppo on Thursday at the start of a "decisive battle" to push President Bashar al-Assad's forces out of the country's biggest city.
Heavy fighting was reported in around 14 districts by opposition activists and several rebel brigades, though the scale of the fighting could not be confirmed independently.
A video posted on YouTube by rebels showed Abdulqadir al-Saleh, the head of the biggest rebel force in Aleppo, the Tawheed Brigade, carrying a walkie talkie and announcing the start of the assault.
"Now the attack on Assad's forces has started on all fronts and God willing today will be decisive in Aleppo," he said.
The 18-month-old uprising against Assad has killed 30,000 people and the violence is escalating sharply, say activists. World powers are meeting at the United Nations but are divided over the crisis, which has descended into civil war.
The U.N. refugee agency said up to 700,000 Syrian refugees may flee the country by the end of the year, nearly quadrupling its June forecast for the exodus from the deepening crisis.
Around 294,000 refugees have crossed into neighboring Jordan, Iraq, Lebanon and Turkey, most of them in the last two months.
"This is a significant outflow taking place, 100,000 people in August, 60,000 in September and at the moment 2,000 or 3,000 per day or night," Panos Moumtzis, Regional Refugee Coordinator for UNHCR, said on Thursday in Geneva.
The flow of refugees surged in August when Assad deployed jets and helicopters to strike rebel-held towns in the northern provinces of Aleppo and Idlib, sending thousands across the border with Turkey.
Rebels flooded into Aleppo two months ago but have been held back by low ammunition and the army's superior firepower.
Insurgents from many brigades in Aleppo said "zero hour" had been announced by calls of Allahu Akbar ("God is greatest") from mosques at 4 p.m. (1300 GMT) on Thursday. They said there were clashes in the heart of the city, including the Hamidiya and Midan districts.
Tawheed Brigade Spokesman Abu Firas said all the battalions working in Aleppo "from the small ones that consist of ten people to the big ones" had met to agree on the offensive two days ago.
The British-based Syrian Observatory for Human Rights, a violence watchdog with a networks of activists in the country, said hundreds of insurgents were taking part in heavy fighting and the government was responding with heavy artillery.
Aleppo is split roughly in half, with rebels in the east and government forces and local Assad-loyal militia in the west.
Syria's rebels are a mixture of army defectors, Islamists and civilians who have taken up arms against Assad.
Some groups have been accused of human rights abuses and western powers who call for Assad's removal are reluctant to support the disparate fighters. Western diplomats say they are looking for a clear command structure and coordination among rebels.
"STAIN ON U.N."
World leaders meeting at the United Nations have expressed concern at the violence but are deadlocked over their response to the conflict, which pits mainly Sunni Muslim rebels against Assad, who is from an Alawite minority close to Shi'ite Islam.
Qatar, one of several Sunni Muslim powers which supports the rebels, called for a no-fly zone to provide a safe haven inside Syria. French President Francois Hollande also called on the global body to protect what he called "liberated zones" under rebel control.
But there is little chance of securing a Security Council mandate for such action given the opposition of veto-wielding members Russia and China.
Russian President Vladimir Putin said in Moscow on Wednesday any attempt to use force unilaterally or interfere with events in the Middle East would be counterproductive. China's Foreign Minister Yang Jiechi said the destiny of the region "should be held in the hands of its own people".
Turkish Prime Minister Tayyip Erdogan said on Thursday Russia, China and Iran's stance on Syria was letting a massacre go on unabated.
Activists said more than 300 people died on Wednesday, making it one of the bloodiest yet. They said 55 people had been killed in rural areas around Damascus, including at least 40 who appeared to have been executed in the town of al-Dhiyabia, southeast of the capital.
The Observatory also said 14 people had been killed in a rebel bomb attack on a military command centre in Damascus and in a prolonged gun battle between rebels and members of the security forces.
The Islamist militant group Al Nusra Front claimed responsibility on Thursday for the assault, saying it was carried out in two stages, starting with a suicide bomber detonating a car near the building.
Rebels have also targeted Assad's inner circle during the conflict. The president's widowed sister Bushra has moved to the United Arab Emirates, a source close to the UAE government said on Thursday, apparently having fled with her children after her powerful husband was killed.
Bushra al-Assad, Bashar's older sister, was married to Assef Shawkat, the former deputy defense minister who was killed in a bomb attack in Damascus on July 18.