Syrian government warplanes and ground forces bombarded Aleppo on Saturday as troops struggle to clear the country's largest city of lightly-armed rebel forces nearly five weeks after they stormed their way into it, activists said.

Meanwhile, rebels seized an air defense installation in the east of the country near the Iraqi border, where opposition forces have claimed advances over the past days, the Associated Press reports.

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, according to AP.

The rebels have increasingly targeted the air force in recent weeks, since it has ravaging densely populated cities with helicopter gunships and fighter jets. Another activist group, the Local Coordination Committees, also said the Bashar al-Assad regime is making heavy use of warplanes in attacking rebel areas.

Both Damascus and Aleppo -- Syria's two largest cities -- have seen persistent fighting between troops and rebels in a conflict that has now claimed more than 26,000 lives since March 2011, according to the Observatory's figures.

In the east, the Observatory reported that rebels overran an air defense building in the town of al-Boukamal in the oil-rich province of Deir el-Zour early Saturday, killing the commander, taking at least 16 captives and seizing an unknown number of anti-aircraft rockets.

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, AP reports.

Russia, Assad's main defender on the world stage, said Saturday that it would be "naïve" for outside powers to expect him to withdraw his troops from cities first and then wait for the opposition to follow suit.

"When our partners say that the government must stop first and withdraw all its soldiers and weapons from cities -- and only then call on the opposition to do the same -- well, this is a completely unworkable scheme," said Foreign Minister Sergei Lavrov. "Either people are naive or it is some sort of provocation."

Turkey has been pressing for the establishment of havens inside Syria to stem the mounting exodus of refugees across its southern border, and reacted with frustration after a United Nations Security Council meeting this week failed to deliver much beyond a French plan to channel more aid to rebel-held areas.

"Bashar al-Assad has come to the end of his political life. At the moment, Assad is acting in Syria not as a politician, but as an element, an actor, of war," Turkish Prime Minister Recep Tayyip Erdogan said in an interview broadcast on Turkish television late on Friday.

Turkey will continue to push for foreign agreement on setting up a safe zone at the U.N. General Assembly next month and will try in the coming weeks to pressure Russia and Iran, who strongly oppose any such action, Reuters reports, citing Turkish government sources.

Meanwhile, Assad's state-run SANA news agency 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.