Syrian rebels withdrew from a prison in the northern city of Aleppo Thursday after heavy fighting with government troops, an activist group said, as it more than doubled its tally of deaths from sectarian killings in a coastal city earlier this month.

The Britain-based Syrian Observatory for Human Rights raised the death toll from the May 3 sectarian killings in the coastal city of Banias to 145 from 62. Activists said at the time that troops and pro-government gunmen stormed the predominantly Sunni Muslim neighborhood of Ras Nabeh and killed dozens.

The violence in the coastal region of Syria underscored the sectarian nature of the two-year conflict, which has killed tens of thousands and forced more than 1 million Syrians to flee to neighboring countries.

Syria's Sunni majority forms the backbone of the rebellion, while President Bashar Assad's minority Alawite sect, an offshoot of Shiite Islam, anchors the regime's security services and the military's officer corps. Other minorities, such as Christians, largely support Assad or stand on the sidelines, worried that the regime's fall would bring about a more Islamist rule.

Rami Abdul-Rahman, who heads the Observatory, said some of the people who have been missing in Banias have turned out to be dead. He said the 145 include 34 children and 40 women.

"This is one of the ugliest massacres that took place in Syria," said Abdul-Rahman, adding that all the 145 killed were civilians. "What happened in Banias was sectarian cleansing."

The killings in Banias came a day after regime troops and gunmen from nearby Alawite areas allegedly beat, stabbed and shot at least 50 people in the nearby Sunni Muslim village of Bayda.

The violence in Banias and Bayda bears a close resemblance to two reported mass killings last year in Houla and Qubeir, Sunni villages surrounded by Alawite towns. Some activists said the Houla and Qubeir carnage, which they blame on regime forces and associated militias, was aimed at driving Sunnis from areas near main routes to the coast in order to ensure Alawite control there.

Abdul-Rahman said some fighters were among the dead in Bayda and Houla.

Another activist group, the Local Coordination Committees, said earlier this month that 102 people were killed in Banias. It said then that some people were still missing.

In Aleppo, the rebel assault at the Aleppo prison began at dawn Wednesday with two simultaneous car bombs detonated at its entrance. By nightfall, the rebels had not dislodged regime forces or freed some 4,000 prisoners held there.

The Observatory said Syrian warplanes bombarded areas around the prison causing casualties among rebels. State news agency SANA denied opposition fighters entered the prison compound, saying regime troops had repelled the attack.

The Observatory also reported that government troops shelled rebel-held northern and southern neighborhoods of the capital Damascus, adding that warplanes carried out at least two air raids on the Damascus suburb of Sbineh.

The Observatory and the LCC said troops also shelled the town of Halfaya in the central province of Hama. Both groups said rebels carried out attacks against regime forces in the town of Khan al-Assal in Aleppo province along Syria's border with Turkey, where opposition forces hold large swathes of land and control whole neighborhoods in Aleppo  city, the country's largest urban center.

In Washington, Turkish Prime Minister Recep Tayyip Erdogan is looking for stepped-up action on Syria as he meets with President Barack Obama Thursday just days after a twin car bombing killed 46 people on the Turkish side of the two countries' common border. Turkey blamed Syrian intelligence for the attacks.

The bombings in the border town of Reyhanli Sunday were the biggest incident of cross-border violence since the start of Syria's bloody civil war, raising fears of Turkey being pulled deeper into a conflict that threatens to destabilize the region.

Like the U.S., Turkey has supported the opposition in Syrian crisis, and Erdogan has been calling for more aggressive steps to topple Assad, including establishing a no-fly zone in Syria. The Obama administration remains reluctant to take the kind of action Turkey would like to see.

While U.S. officials have said they are not excluding the possibility of arming some carefully vetted groups of rebels in the future, they remain reluctant of providing the opposition with heavier weapons for fear they could end up in the hands of radical Islamic groups that have become the most effective fighting force on the opposition side.

Syria's crisis, which began in March 2011 with pro-democracy protests and later turned into a civil war that has killed an estimated 70,000 people, has taken on increasingly sectarian overtones.