NEW: The U.N. secretary-general urges Syria to stop its offensive
Am assault is reported in the Fardos district in Aleppo
Aid agencies are scaling back in Aleppo
Deaths mount across Syria on Friday, the opposition says
Fighting engulfed parts of Aleppo on Friday as world powers issued warnings of a government onslaught in the sprawling and densely populated Syrian city.
The opposition Local Coordination Committees of Syria reported a "horrific massacre" in the city's Fardos district, where regime forces indiscriminately shelled homes.
The international activist group Avaaz, saying it has spoken to witnesses, also reported violence in Fardos.
The witnesses reported casualties in a "blistering" regime "dawn attack" in the district, in central Aleppo. They told Avaaz that the regime used mortars, tanks and helicopter gunships in the bombardment.
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Regime forces in the Khaldieh district fired at protesters coming out of the Al-Ghafran mosque and shelled the Salaheddine district with missiles, automatic weapons and attack helicopters, the LCC said.
Syrian state TV said that "special security authorities" destroyed five pickup vehicles equipped with machine guns used by "terrorists" in the Aleppo countryside. The report said people in the vehicles were killed and injured.
At least 11 of the 70 people slain in Syria on Friday were from Aleppo province, the LCC said, and the count is expected to rise. The day before, 48 of the 200 people killed in Syria violence were also from the Aleppo region, the first time since the uprising started that the region led in the number of deaths in a single day.
As the violence unfolded, throngs of protesters marched down Aleppo streets Friday, chanting and carrying signs, saying, "we stand for all the Syrian devastated cities" and "we are all Syrians."
The world community urged Bashar al-Assad's regime to stop its assault.
"I am seriously concerned by the escalating violence in Aleppo, Syria," U.N. Secretary-General Ban Ki-moon said on Friday in London. "I urge the Syrian government to halt their offensive. The violence from both sides must stop for the sake of the suffering civilians of Syria."
British Foreign Secretary William Hague said this "utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster. It will add to the misery being endured by the Syrian people, and plunge the country further into catastrophic civil war."
Bernard Valero, a French Foreign Ministry spokesman, said his country fears that "more and more civilians will be killed in Aleppo" as military forces circle the city and prepare to launch a "devastating" military operation.
"Bashar al-Assad is about to commit another horrific crime against his own population," he said.
Navi Pillay, the U.N. high commissioner for human rights, said the buildup of forces in and around Aleppo "bodes ill for the people of that city" in light of the carnage across the nation.
U.S. State Department spokeswoman Victoria Nuland on Thursday expressed "grave concerns" that Syrian forces were preparing to carry out a "massacre" in and around the city. She noted that the Aleppo region has been bombarded by Syrian fighter jets and that columns of regime tanks are reportedly prepared to attack.
Rebels braced for a fight. One rebel commander said plans were under way to send 300 more fighters to bolster forces in the city, where 18 of 22 rebel brigades are located.
Another, Mustafa Abdullah, told CNN that rebels have set up medical clinics in Aleppo homes and have plans to transport and evacuate anyone who is wounded.
"They (government forces) want to surround Aleppo completely and send support from all sides, then start shelling rebel-controlled areas and hospitals," Abdullah said.
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Aleppo, in northern Syria near Turkey, is the country's commercial capital and a World Heritage site. The CIA World Factbook says it is the nation's most populous city.
Rebel militias battling regime forces in Aleppo and other hotspots are composed largely of soldiers who have defected from the Syrian military. But there are also many civilians -- including students, shopkeepers, real-estate agents and members of the president's ruling Baath party -- all trying to end four decades of Assad family rule.
Abdullah, the rebel commander in Aleppo, recalled the merciless government bombardment of the city of Homs this year as he geared up for battle. Asked if these fighters had enough ammunition to withstand a government siege, Abdullah said simply "no." He then added, "It will be just like Homs" and wept at the thought.
The Aleppo fighting has political significance.
A Syrian parliamentarian from Aleppo has defected to Turkey, according to the opposition Syrian National Council. Ikhlas Badawi is the first member of the assembly elected in May to defect and the latest in a series of high-profile officials to cut ties with the regime. This follows defections by high-level Syrian diplomats to the United Arab Emirates, Cyprus and Iraq.
Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy, said that if the government military effort fails in Aleppo, "it will cost the regime dearly and begin a process of contraction of regime control over Syria."
"If the regime is able to beat the rebels back, this conflict will go on far longer," he said.
As fighting raged, aid agencies scaled back their efforts in Aleppo.
A spokeswoman for the International Committee of the Red Cross -- which said it is temporarily moving some aid workers out of Syria to Beirut for security reasons -- said the organization doesn't have a presence in Aleppo at the moment. In addition, the Syrian Arab Red Crescent has had to suspend some of its activities.
"The situation there is extremely volatile," the ICRC said. "Several schools in Aleppo have been opened to host displaced families, and the local SARC volunteers have provided some relief items (mattresses, hygiene products and food). However, humanitarian needs are on the rise."
Outside Aleppo, battles between regime forces and rebels raged. Heavy shelling rocked other cities, including Damascus, Daraa, Idlib and Homs, the Local Coordination Committees of Syria said. Twenty-one people died in Daraa, 15 in Damascus and its suburbs, and 10 in Homs, the LCC said.
People fleeing violence were on the move from Aleppo and other hotspots. A group of Syrian refugees came under fire Thursday night while crossing the border into Jordan, said Petra, the Jordanian state news agency. A child was killed during the incident, according to Samih Maaytah, the minister of state for media affairs.
The Syrian crisis started in March 2011 when a government crackdown on peaceful protesters morphed into a nationwide uprising against the regime.
The LCC says more than 16,000 people have been killed in the conflict. The U.N. secretary-general said this week that almost 17,000 people have died.