DAMASCUS, Syria, July 23 (UPI) -- Emboldened Syrian rebels fought regime troops over control of Syria's biggest city, Aleppo, while other rebel forces retreated under heavy fire in Damascus.
The broadened fighting to Aleppo, a northern city of 2 million that is Syria's commercial and business hub, came as intelligence officials said Washington was mounting a secret but limited effort to speed the fall of Syrian President Bashar Assad without using force.
It also came as Arab League foreign ministers called in Assad to "renounce power," promising "a safe exit" for him and his family.
Free Syrian Army Col. Abdul-Jabbar Mohammed Akidi, one of the rebel commanders, said in an opposition video released on YouTube Sunday the "Brigade of Unification" sought to free Aleppo from the Assad regime.
"We gave the orders to march on Aleppo with the aim of liberating it," he said in the video.
Reached by Skype, Akidi told the Los Angeles Times his forces controlled the Salahudeen, Sakhoor and Tareeq al-Bab neighborhoods, and had repulsed a counterattack by regime forces, killing 20 soldiers.
Official state media reported authorities in Aleppo had inflicted "heavy losses" on "terrorists," the government's term for the armed rebels.
Rebel officials warned people to stay in their homes. Many residents were reported fleeing the city.
In Damascus, the Syrian capital, opposition activists reported a new wave of regime attacks involving heavy artillery, tanks and helicopter gunships targeting opposition strongholds.
Residents said Free Syrian Army forces retreated but sought to reclaim their positions after heavy aerial and ground bombardment in the Mezzeh neighborhood, one of Damascus' most modern and expensive areas that is home to many embassies and offices.
Activists said regime troops executed at least 20 unarmed men in Mezzeh Sunday who they suspected of aiding rebels.
The Local Coordination Committees activist network said "many individuals were injured" when rocket and helicopter shelling damaged and burned the Grand Mosque in Damascus' Barzeh neighborhood.
The state-run Syrian Arab News Agency said, "Authorities, in cooperation with locals, are chasing vanquished terrorists in the streets."
SANA dismissed as "baseless" opposition claims of aerial bombardment.
The Damascus provincial governor was quoted vowing the city's battered Midan district -- site of intense fighting last week -- would be back to "normal life" by Friday.
Until a week ago, Damascus and Aleppo had been largely untouched by the violence roiling Syria's provinces.
The Arab League foreign ministers, ending a meeting in Doha, Qatar, called on Assad to swiftly give up power to end his country's unrest,promising safety for him and his family.
"There is agreement on the need for the rapid resignation of President Bashar Assad," Qatari Emir Sheik Hamad bin Khalifa al-Thani told reporters in the early hours Monday.
The league urged the Free Syrian Army to form a transitional national unity government.
The Wall Street Journal reported Monday the Obama administration was trying to hasten Assad's fall by blocking arms and oil shipments from Iran and passing intelligence to front-line allies -- but not using force.
The effort, which was recently ramped up as Assad appeared more desperate, involves the CIA, State and Treasury departments and the U.S. military, intelligence officials told the newspaper.
The effort's No. 1 goal, the officials said, was to get Iraq to close its airspace to Iran-to-Syria flights that U.S. intelligence determined were carrying weapons to Assad loyalists -- although flight manifests said the planes were carrying cut flowers.
Some weapons flights were stopped, officials said, but others slipped through, the officials said.
Washington also tried to keep ships believed carrying arms and fuel for Syria from crossing the Suez Canal, with mixed results, the Journal said.
House intelligence committee Chairman Mike Rogers, R-Mich., told the Journal the effort "stops far too short of really having an impact" because there are so many ways of getting arms into Syria, including through Lebanon smuggling routes.
White House National Security Council spokesman Tommy Vietor declined to comment on specific efforts, but said, "It's clear that the Assad regime is losing control of Syria," pointing to a bombing in Damascus last week that killed top Assad advisers.