In the latest clashes, Aleppo's historic Citadel, part of a UNESCO-listed world heritage site, was heavily damaged by bombing, the opposition said.
The violence raged on as world powers prepared to name veteran Algerian diplomat Lakhdar Brahimi as their new envoy to seek an end to a 17-month uprising that has cost more than 21,000 lives.
Washington, meanwhile, upped the pressure on Damascus by imposing fresh sanctions on the regime and on the Lebanese militant group Hezbollah, a close ally of Damascus and Tehran.
A rebel commander, Hossam Abu Mohammed, said his men were still fighting in parts of Aleppo's southwestern district of Salaheddin after most fled on Thursday in the face of heavy bombing and advancing troops.
"We will not let Salaheddin go," the Free Syrian Army's Abu Mohammed told AFP by telephone on the third day of a government offensive to take the city.
And one of the fighters told AFP the rebels were keeping at bay troops who control a key roundabout from advancing further into Salaheddin.
"They have a few soldiers at the roundabout and some snipers. What we are doing now is preventing the troops from advancing," the fighter said on condition of anonymity.
The army again bombed parts of Salaheddin, as well as the Sakhur and Hanano districts in the northeast, the Syrian Observatory for Human Rights said.
At least 103 people were killed nationwide on Friday, including 55 civilians, the watchdog said. One of those killed was a 19-year-old protester shot dead by regime forces in Aleppo.
In the central city of Homs, the army pounded the rebel stronghold of Khaldiyeh with "dozens" of people killed or wounded, the Observatory said.
Before dawn, a MiG 21 fighter jet dropped four bombs on rebel positions in Hanano, an AFP correspondent said. One struck the courtyard of an FSA compound and another struck a nearby house, wounding a number of people.
Angry residents shouted hostile slogans against France and the United States, saying: "No one is helping us."
"We are behind the Free Syrian Army, but it is because of them that all of this is happening," one of them lamented.
The opposition Syrian National Council said Aleppo's 13th-century Citadel, part of a complex of sites in the city's historic heart that the UN Educational, Scientific and Cultural Organisation says is of "outstanding universal value," had been damaged in army shelling.
It was not possible to independently verify the claim.
Also on Friday, rebels captured three journalists who work for Syrian public television Ikhbariya as they accompanied government troops operating near Damascus, the Observatory said.
Ikhbariya later said it had lost contact with its crew.
-- 'Syrians clamouring for peace' --
In a bid to starve the regime of much-needed revenue, the United States slapped sanctions Friday on the Syrian state oil marketing company Sytrol for trading with Iran.
And the US Treasury said it was adding Lebanon's Hezbollah to a blacklist of organisations targeted under Syria-related sanctions.
"Iranian officials have boasted about Iran's support to Assad," Ventrell said. "Iran's actions in Syria underscore its fear of losing its only remaining ally in the Middle East and an important conduit to Hezbollah."
Meanwhile, diplomats at the United Nations said former Algerian foreign minister Brahimi was expected to be named as the new UN-Arab League envoy to Syria early next week.
Kofi Annan, a former UN secretary general, resigned from the post of envoy, saying he had not received enough international support for his efforts to end the conflict but is staying on until August 31.
In a statement released by The Elders, a group of world statesmen, Brahimi said "the UN Security Council and regional states must unite to ensure that a political transition can take place as soon as possible."
"Millions of Syrians are clamouring for peace. World leaders cannot remain divided any longer, over and above their cries."
Britain said on Friday it would give the rebels five million pounds ($7.82 million, 6.3 million euros) in non-lethal assistance, including body armour and communications equipment.
Meanwhile the International Committee for the Red Cross said the Syrian Red Crescent had suspended most of its work in Aleppo because of the extreme danger.
A statement in Geneva said the ICRC had managed on Thursday to deliver food and other essentials to cover the needs of at least 12,500 people in the city of some 2.7 million people.
Meanwhile the imam of the Grand Mosque in Mecca, Islam's holiest site, warned that a "genocide" was underway in Syria and urged participants at an Islamic summit to take place on Tuesday to take action against the Syrian regime.
Mosques and Islamic centres across the United States also came together on Friday to condemn the Syrian regime's brutal crackdown and raise funds for civilians trapped in the conflict.