This image supplied by Shaam News Network purportedly shows bodies of Syrians killed in the Damascus suburb of Daraya, Tuesday, July 24, 2012. / AP Photo/SNN
Last Updated 12:27 p.m. ET
(CBS/AP) BEIRUT - A showdown between rebels and government troops in Syria's largest city, Aleppo, is imminent, the U.N.'s human rights office said Friday, as the Red Cross pulled some of its foreign staff from Damascus out of concern for the safety of its workers.
Syrian rebels have made a run on the country's two biggest cities, Aleppo and Damascus, since last week. Regime forces have responded with overwhelming firepower, ushering in some of the most serious violence the cities have seen in 17 months of conflict.
Rebels have been locked in fierce fighting with government troops in Aleppo for seven days and they are bracing for an attack amid reports that the regime is massing reinforcements to retake the embattled city of 3 million.
At a press conference in London, U.N. Secretary-General Ban Ki-moon demanded that Syrian authorities "categorically state that they will not use chemical or other weapons of mass destruction under any circumstances."
CBS News correspondent Clarissa Ward reports that fighters who call themselves the Sham Falcons have accomplished something very significant: Using light weapons, rocket-propelled grenade launchers and homemade bombs, they have pushed Syrian government forces back, and carved out an area of rebel control in the hills of Jabal al-Zawya.
The stark landscape is pock-marked with the scars of heavy fighting. But there are no Syrian Army troops left in this area. To opposition members, it is known as Free Syria.
Abu Issa and his men - he claims to have between six and seven thousand fighters - fought inch by inch to hold this territory. But holding territory is no longer enough. Now they're taking the battle forward.
These rebels still have a long way to go, Ward says: The Syrian military is throwing everything it has at the battle in Aleppo, and it is sure to be a long and bloody fight.
To watch Clarissa Ward's report click on the video player below.
Mohammed Saeed, an Aleppo-based activist, said helicopters were firing with heavy machine-guns on rebel-held areas east and west of the city on Friday. He added that army reinforcements arrived in the city on Thursday and a major attack is expected any time.
U.N. High Commissioner for Human Rights Navi Pillay said unconfirmed reports are coming out of the capital, Damascus, of extra-judicial killings and shootings of civilians during fighting in the city's suburbs. Pillay said the report "bodes ill for the people of that city (Aleppo)."
Pillay said she believes President Bashar Assad's regime and opposition forces are both committing crimes against humanity and war crimes.
"And it goes without saying, that the increasing use of heavy weapons, tanks, attack helicopters and reportedly even jet fighters in urban areas has already caused many civilian casualties and is putting many more at grave risk," she said in a statement read aloud to reporters by her spokesman Rupert Colville.