BEIRUT (AP) — Backed by warplanes, Syrian government troops pushed Thursday into the central districts of the city of Homs in an effort to oust rebels from the country's third largest urban center, activists said.

The regime pounded rebel-held districts with artillery and carried out at least one airstrike on a residential neighborhood, killing seven people, four of them children, said the Britain-based Syrian Observatory for Human Rights.

President Bashar Assad's troops regained control of the Wadi Sayeh district in the center of Homs early Thursday morning, the Observatory said. The neighborhood is strategically important for the government as its forces try to dislodge opposition fighters from several central districts that have been under rebel control for more than a year.

In the northern city of Aleppo, rebels overran the headquarters of Assad's anti-terrorism forces, according to the Aleppo Media Center activists group. The building's capture was the latest in a string of rebel victories in Syria's largest city, where opposition fighters are trying to expand their hold.

The building is located near the central prison where many of Assad's opponents, activists and their family members are believed to be held. Rebel fighters have for weeks battled government troops in the area in an attempt to storm the facility and free the prisoners.

Both Homs and Aleppo, along with the capital of Damascus, have been key battlegrounds in the country's conflict, which is now in its third year and has claimed more than 70,000 lives. In the past year, the fighting has taken increasingly sectarian overtones. Members of Syria's Sunni Muslim majority dominate the ranks of the rebels fighting to topple Assad's regime, made up mostly of the minority Alawite sect — an offshoot group of Shiite Islam that the president's family also belongs to.

The Observatory, which relies on a network of activists on the ground, said regime jets also hit the northern city of Raqqa, killing eight people there on Thursday. In early March, Raqqa became the first urban center to fall entirely under rebel control.

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Also Thursday, fierce clashes broke out near the coastal town of Banias, the Observatory said, adding that at least six government soldiers were killed in the fighting and 20 other were wounded. Banias is a predominantly Sunni town on the Mediterranean, although the broader coastal area is an Alawite heartland.

The Syrian conflict started with largely peaceful protests against Assad's regime in the impoverished suburbs of the major cities, but quickly spread around the country, descending into a full-scale civil war.

In recent weeks, government forces have been on a counter offensive to reverse rebel gains in and around Aleppo, where the opposition controls whole neighborhoods and large swaths of land around the city.

The army has also conducted massive sweeps through opposition strongholds around Damascus to oust rebels from their enclaves, from where they are increasingly threatening the capital, the seat of Assad's power.

Regaining full control of Homs would be a psychological blow to the opposition, which considers the city a symbol of Syria's uprising, inspired by other Arab Spring revolts against authoritarian rulers around the Middle East.

Homs was the scene of massive street protests against Assad's regime in the first months of the uprising that began in March 2011. Since then, it has seen some of the worst urban warfare in the country's civil war.

Also Thursday, Assad's warplanes hit several targets in Damascus province, including rebel positions in neighborhoods around the capital, as well as targets in the southern city of Daraa, the Observatory said.

Opposition activists also reported fierce clashes between Assad's troops and rebels near Aleppo's international airport. Aleppo, Syria's main commercial hub, has been carved up into opposition- and government-held areas since the rebels launched an offensive there last summer, capturing several districts and villages and towns around the city, which is near the border with Turkey.

On the frontier, a group of Syrians attacked Turkish border officials, burned sentry boxes and fired automatic rifles randomly toward Turkey after being refused entry into the country, Turkish officials said. Seven people, including three Turkish police officials and two soldiers were wounded, said Adulhakim Ayhan, mayor of the border town of Akcakale, on private NTV television.

Syria's state-run SANA news agency said three people were wounded in a mortar attack on the northern Damascus district of Abbasid Thursday. The district is located near the neighborhood of Jobar, from where rebels have been trying to push into the capital in the past weeks. In another part of Damascus, a bomb attached to a car detonated, wounding one man and causing material damage, SANA said.

Earlier this week, two massive blasts went off in central Damascus, which has so far been spared the scale of destruction that Aleppo and other cities have sustained in the war.

On Tuesday, a powerful bomb rocked a busy commercial district, killing 14 people, the Syrian state TV said. A day earlier, Syria's prime minister narrowly escaped an assassination attempt after a car bomb struck near his convoy in a nearby area in Damascus.

The bombings appear to be part of an accelerated campaign by opposition forces seeking to strike at Assad's heavily protected seat of power.

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Associated Press writer Bassem Mroue in Beirut contributed to this report.