Syria's most populous city remained engulfed by fighting Tuesday as opposition groups reported incremental rebel gains and the United Nations said civilian displacement rose.

Fighters attacked Syrian police stations in the central neighborhoods of Salhin and Bab al-Nayrab, and seized control of the buildings after hours of clashes, the opposition Syrian Observatory for Human Rights reported.

At least 40 police officers died in the violence, the observatory said. Deama, an activist who asked that her full name not be used for her protection, said Salhin has been the center of many "aggressive operations by the police and Shabiha militia."

A opposition video, purportedly from the Bab al-Nayrab neighborhood, showed bloody corpses amid rubble and rebels chanting "Allahu akbar," an Arabic phrase for "God is great."

Amid spurts of gunfire, emboldened and elated fighters also shouted "Hafez Assad, the dog of the Arab Nation" and "the Free Syrian Army forever, stepping on Assad's head."

Hafez Assad, the late leader of Syria, is the father of Syrian President Bashar al-Assad. The Free Syrian Army is an anti-government fighting force.

One rebel who identified himself as Faris said Shabiha militia and rebels were fighting in Bab al-Nayrab.

Among those fighting for the regime, he said, are people from the well-known al-Berri clan, who have members in parliament. A lot of FSA fighters died in the clashes, Faris said.

Deama said rebels remain in control of the neighborhood of Salaheddine and battled regime forces there Tuesday. The area is in the southwestern part of the city.

Even though clashes engulfed Salaheddine, helicopter shelling had not been reported Tuesday, prompting speculation from opposition people that the regime might be planning a big push in the area, Deama said.

Free Syrian Army deputy commander Malek Kurdi, now in rural Aleppo, said the regime has been trying to storm Salaheddine, but has been regularly repelled by the FSA. He said he's seen the regime forces use rockets, 130 mm shells and 120 mm mortars.

"The situation is good now," Kurdi told CNN in a phone call. "I think the regime's troops are too scared to go in. Eight tanks and BMPs have been destroyed by our fighters in the first attempt by the regime to overrun the neighborhood. The regime forces tried to go in today as well, but they couldn't."

"BMP" is the Russian and Arabic acronym for armored personnel carrier.

Also, he said, FSA fighters overran a big security checkpoint in the town of Anadan just north of the city two days ago and the rebels are working to take control of Minakh military airport in Aleppo.

"If that is achieved successfully, then we can say the entire northern and northeastern part of Aleppo will be liberated," he said.

Kurdi said he believes the rebels are geared up for government strikes from three directions.

"The regime is bringing infantry reinforcements by helicopters to the civilian airport in Aleppo and to Ramousa in Aleppo," he said. "But we are ready, and the FSA fighters are already making advances."

Elsewhere in Aleppo, helicopters fired rockets at several neighborhoods, including Maisar, Bab Road, Ard Hamra, Sakhour and Karm Almuyassar, opposition sources said.

The Syrian Observatory said a sniper killed the leader of a rebel battalion in the Marjeh neighborhood, and the opposition Local Coordination Committees of Syria said the Free Syrian Army and regime soldiers were battling in the Meredian neighborhood.

State-run TV said Syrian forces clashed with "armed terrorist groups" on the outskirts of Aleppo and destroyed nine armored vehicles "with all terrorists inside."

"Our armed forces continue to pursue terrorists in the Salaheddine neighborhood in Aleppo. The operations resulted in inflicting heavy losses among the terrorists and the confiscation of their weapons," state TV said.

Aleppo is the commercial and cultural hub of Syria, and the fight to seize control of the urban center is a major battle in what world powers now regard as a civil war.

The fighting comes a day after rebels scored a notable victory when they captured an army outpost near Aleppo, taking possession of tanks and crates of ammunition in the process.

Unrest spread across other volatile regions of the country Tuesday, as regime forces shelled targets and launched raids in and around Damascus, Homs, Daraa and Deir Ezzor.

The LCC said at least 49 people have been killed in these regions of Syria on Tuesday. At least 20 of them died in Aleppo. It is not clear if any of them were among those killed in the siege of the police stations.