Syrian warplanes and ground forces pounded the country's largest city Aleppo with bombs and mortar rounds on Saturday as soldiers clashed with rebels in its narrow streets, activists said.

The latest violence shows that government troops are still struggling to clear the city of lightly-armed rebel forces nearly five weeks after they stormed their way into it.

Activists also said rebels captured an air defense facility in the east of the country near the border with Iraq, where opposition forces have claimed advances over the past days.

The Britain-based Syrian Observatory for Human Rights said the clashes in Aleppo were concentrated in several tense neighborhoods — Hanano, Bustan al-Qasr, Sukkari and Maysar. It reported injuries and damage to buildings.

Another activist group, the Local Coordination Committees, said the government was making heavy use of warplanes in attacking rebel areas.

A video obtained by The Associated Press Television News showed rebel fighters, some in civilian clothes, in the street trading fire with government troops.

Activists say that this is the second day of a rebel push in Aleppo dubbed "Northern Volcano" targeting security facilities in the city and the surrounding province, including an artillery training school, a compound of the feared air force intelligence, and a large army checkpoint.

For over a year after the uprising against President Bashar Assad regime began in March 2011, Aleppo and Damascus stayed relatively quiet. But in July, rebels launched a brazen attack on the two cities, capturing several neighborhoods.

Government forces have regained most of the Damascus area but are being held at bay in Aleppo.

In the east, the Observatory reported that rebels captured an air defense post in the town of al-Boukamal in the oil-rich province of Deir el-Zour that borders Iraq. A video released by activists showed soldiers who said they were captured at the post after rebels took it. The authenticity of the video could not be independently confirmed.

The fighting comes as veteran U.N. diplomat Lakhdar Brahimi is due to begin a peacemaking mission, replacing Kofi Annan who quit after his six-point plan including an April 12 cease-fire failed to stop the bloodshed.

Unlike Annan, who was based in Geneva for six months, Brahimi will make his center of operations in New York, where he hopes he can better influence the U.N. Security Council to unite around a plan to end the violence.

Russia and China used their vetoes on the council to block U.N. sanctions against the Syrian regime, despite entreaties by the U.S. and other Western nations.

Annan blamed divisions on the 15-nation Security Council for the failure to persuade Assad and the opposition to end the conflict.

Brahimi, a former Algerian foreign minister who has been a U.N. envoy to Afghanistan and Iraq, said his first task will be to overcome the divisions in the Security Council and get it to speak "with a unified voice."

Activists say more than 20,000 people have been killed since the beginning of the uprising, which eventually became a civil war.

Despite the government's control of Damascus, opposition fighters continue to stage attacks using hit-and-run tactics in neighborhoods where they enjoy popular support, activists say.

Early Saturday, government forces bombarded the capital's southern neighborhood of Tadamon followed street fighting with rebels there, the Observatory said. The LCC said troops also shelled the nearby neighborhood of Hajar Aswad.

The state-run news agency SANA said army Brig. Gen. Taher Subeira was killed by "terrorist" who placed a bomb under his car parked in front of his Damascus home and detonated it when he got into the vehicle.

The Observatory said the bodies of five unknown people were found in the neighborhood of Qadam, shot execution-style.

Other clashes were reported in Idlib province on the border with Turkey, in Daraa near the Jordanian frontier, and in the central province of Homs near Lebanon, activists said.

SANA meanwhile reported Saturday that 225 detainees who took part in anti-government protests were released. The amnesty by authorities is the second in a week as some 378 prisoners from Damascus and the central province of Homs were freed on Monday.

Rights activists say tens of thousands of Syrians have been detained over the past 18 months.