Some of the heaviest fighting erupted in the predominantly Kurdish neighborhood of Sheikh Maksoud, which was drawn into the conflict for the first time. Kurds make up Syria's largest minority and have been split in their loyalties.
Since the uprising against Assad began 18 months ago, some Kurds have sided with the rebels while others have supported the regime. Aleppo's Sheikh Maksoud neighborhood is mostly under the control of a pro-government Kurdish group. There were conflicting reports on whether Kurdish gunmen took part in Friday's fighting.
Aleppo, a city of 3 million that was once a bastion of support for Assad, has emerged as a key battleground in Syria's civil war. Its fall would give the opposition a major strategic victory, with a stronghold in the north near the Turkish border. A rebel defeat, at the very least, would buy the regime more time.
"The city is witnessing one of the most violent days. All fronts are on fire," Aleppo-based activist Baraa al-Halabi said.
In the diplomatic arena, U.S. Secretary of State Hillary Rodham Clinton was hosting representatives of the Friends of Syria group—a coalition that includes the United Sates, the European Union and the Arab League—on the sidelines of the U.N. General Assembly. The group is seeking better cooperation among the groups that oppose Assad.
And in Geneva, the U.N.'s top human rights body voted Friday to extend the mission of its independent expert panel probing alleged war crimes in Syria. The panel led by Brazilian professor Paulo Sergio Pinheiro has blamed Syria's government forces for the majority of serious abuses since the uprising began in March 2011.
The two sides have been locked in a stalemate, with neither able to deliver a decisive blow. The standoff has been most apparent in Aleppo, where each side controls roughly half the area. Late Thursday, rebel commanders from the main group of fighters, the Free Syrian Army, announced they are embarking on what they called a "decisive battle" for the city.
Since then, heavy clashes have been reported along the front lines between rebel- and regime-held areas.
Al-Halabi said clashes had broken out in neighborhoods including Midan, old Aleppo, Maysaloun, Azamiyeh, Salaheddine, Seif al-Dawla and Sheikh Maksoud.
Al-Halabi said the fighting included members of a group affiliated with the Kurdistan Workers' Party, or PKK, which is fighting for autonomy in the Kurdish-dominated southeast region of neighboring Turkey and has had close ties with the Syrian government for decades
Another Aleppo-based activist, Mohammed Saeed, said the Kurdish fighters withdrew shortly after the fighting began without taking part in the battle.
The PKK's affiliate, the Democratic Union Party, is active in Sheikh Maksoud.
The Tawhid Brigade, the major rebel group in the city, said on its Facebook page that some of its fighters were battling pro-PKK gunmen in the neighborhood. State-run Syrian TV said regime forces repulsed an attack on Sheikh Maksoud with the help of its residents.
Rami Abdul-Rahman, who heads the Britain-based Syrian Observatory for Human Rights, also said Kurds did not participate in the battle. He said those fighting alongside the regime in Sheikh Maksoud were members of a local Syrian clan known to support Assad.
The reports could not be independently confirmed because the government has imposed tight restrictions on the media.
Both the regime and the opposition have courted the Kurds, Syria's largest ethnic minority. Kurds make up around 10 percent to 15 percent of the country's 23 million people.
In July, government forces withdrew from Kurdish areas in northeastern Syria and were quickly replaced by Kurdish fighters from the PKK affiliate. However, other Kurds have sided with those trying to topple the regime, complaining of neglect by the state.
Despite the escalating bloodshed in Syria, the international community remains sharply divided over how to end the conflict.
The U.N. Security Council is deadlocked, with Syria allies Russia and China having repeatedly blocked harsher measures against the Assad regime sought by Western nations and the rebels' allies in the Middle East.
Clinton has decried Assad's "murdering of his own people," while Russia's Foreign Minister Sergey Lavrov—who will address the General Assembly on Friday—has accused the U.S. and other countries of encouraging terrorism in their stance on Syria.
Syria's conflict has repeatedly spilled over into neighboring countries, including Turkey, one of the biggest backers of the rebels.
A mortar round fired during fighting Friday in the area of a Syrian-Turkish border crossing, Tel Abyad, landed in Turkish territory, damaging the walls of two houses in the town of Akcakale, its mayor, Abdulhakim Ayhan, told the state-run Anadolu Agency. No one was hurt. Anadolu said Assad's forces were firing mortar rounds on rebel positions in a bid to regain control of the border crossing.
Also, a Syrian activist group, the Local Coordination Committees, said a Syrian warplane bombed the northern town of Azaz near the Turkish border, killing seven people.
An amateur video showed at least one dead boy being removed from under the rubble of a house in the area.
Associated Press writer Suzan Fraser in Ankara, Turkey, contributed reporting.