"It's warplanes against Kalashnikovs, tanks fighting against rifles," he said. "I don't know how long this situation can be sustained."
As Syrian soldiers bombarded rebel positions in Aleppo from the ground and air, diplomats said former Algerian foreign affairs minister and longtime U.N. official Lakhdar Brahimi has emerged as a strong candidate to replace Kofi Annan as peace envoy to Syria.
Annan announced his resignation last week, ending a frustrating six-month effort that failed to achieve even a temporary cease-fire as the country descended into civil war. Activists say some 20,000 people have been killed in the conflict.
Also Friday, the U.S. announced sanctions on Hezbollah for providing support to Syrian President Bashar Assad's regime a symbolic move, as Washington already has designated the Lebanese militant group a terrorist organization.
Still, the sanctions emphasized how Syria's close ties to Hezbollah and to the group's patrons in Iran mean that the conflict has the potential to escalate dramatically.
The relentless violence triggered a fresh wave of civilians streaming across the border into neighboring Turkey. Turkish officials said more than 1,500 Syrians arrived over the past 24 hours, increasing the number of refugees in Turkey to about 51,500.
The regime has been trying to drive rebels out of Aleppo, Syria's largest city, for more than two weeks. The state news agency claimed Wednesday that Assad's forces had regained control of the Salaheddine neighborhood, the main rebel area in Aleppo. But activists said rebels were still putting up a fight there on Friday despite being low on ammunition.
Aleppo holds great symbolic and strategic importance. Some 25 miles (40 kilometers) from the Turkish border, it has been a pillar of regime support during the uprising against the Assad regime. An opposition victory there would allow easier access for weapons and fighters from Turkey, where many rebels are based.
An Aleppo-based activist said government forces were shelling rebel-controlled areas in the southwestern part of Aleppo and in the northeast. Towns and villages in Aleppo suburbs were "at the mercy" of fighter jets and helicopters strafing the area, he said.
"Soon there will be nothing left to destroy in Aleppo ... The regime is using air power without shame," he said, asking that his name not be used out of fear for his personal safety.
Protesters across many parts of the country rallied after midday prayers Friday, urging the international community to arm the opposition fighters.
"Give us anti-aircraft guns. Where is your conscience?" read a small poster held by a protester in the village of Kfar Zeita in the central Hama province.
But there has been deep reluctance to openly arm the fighters, out of fears that it could escalate the violence and because the rebel Free Syrian Army is not a unified group. Many rebel groups operate largely independently of each other, in many cases sharing only the goal of toppling Assad.
On Friday, Britain's government said it is giving an extra 5 million pounds (US$7.8 million) worth of aid but no weapons to Syria's opposition.
Foreign Secretary William Hague said the funds would pay for items including satellite phones, power generators and medical kits.
Britain has previously given 1.4 million pounds (US$2.2 million) worth of nonlethal support to Syria's opposition. The United States has earmarked a fund of $25 million to spend on nonlethal communications assistance.
The activists, from the Britain-based Syrian Observatory for Human Rights as well as Syria's Local Coordination Committees, also reported shelling Friday of several areas just outside Damascus, where rebels also were active. Residents reported hearing loud blasts in Damascus from the shelling on the outer edges of the city, the activists said.