BEIRUT -- Pro-government gunmen have kidnapped more than 300 people in northwestern Syria in retaliation for the abduction of 42 Shiite Muslims this week, a move that could fuel more sectarian violence in the country, an activist group said Saturday.

The tit-for-tat kidnappings point to the dark sectarian overtones of Syria's civil war, which pits a predominantly Sunni Muslim rebellion against a regime dominated by President Bashar Assad's minority Alawite sect, an offshoot of Shiite Islam. The country is also home to Christian, Kurdish, Armenian and Shiite communities, all of whom have been swept up in the conflict.

The Britain-based Syrian Observatory for Human Rights said the spate of kidnappings this week took place in the northern province of Idlib, which borders Turkey.

While many of the details remain murky, the abductions appeared to have a sectarian bent. Kidnapping for ransom has been widespread across Syria since the crisis began in March 2011, but sectarian and political abductions have been rare.

The Observatory said the 42 Shiites, mainly women and children, were snatched Thursday from a bus that was travelling from the Shiite villages of Foua and Kfarya to the capital Damascus. Observatory director Rami Abdul-Rahman, said it was not clear who took them, adding that Shiites have refused to give the names of those kidnapped or details about the bus.

Idlib-based activist Fadi al-Yassin Al-Yassin said Foua and Kfarya are being used by the regime to bombard nearby villages and towns, saying the regime has turned them into "castles of shabiha," referring to pro-government gunmen.

In retaliation for the bus kidnappings, members of the pro-government Popular Committees set up a checkpoint around the two Shiite villages and on Thursday and Friday were taking people from cars they stopped, the Observatory said. It added that most of the people abducted were from the Sunni villages of Saraqeb, Binnish, Sarmin, Qimnas, Maaret al-Numan and Maaret Musreen.

Al-Yassin confirmed the kidnappings on both sides but added that the 300 figure is high. He said few dozens of people have been abducted in the area.

Abdul-Rahman and al-Yassin said such acts could incite sectarian clashes between Shiites, who have largely sided with the regime, and majority Sunnis in Idlib, where the sects have coexisted for decades.

The high number of women and children allegedly taken prompted the U.N. Special Representative on Sexual Violence in Conflict, Zainab Hawa Bangura, to issue a statement calling for their release.

"Allegations of abduction and rape of women and girls by armed groups have been received," she said. "I am deeply concerned about the well-being of these women and children and would like to remind the armed group responsible for this abduction that acts of sexual violence will not be tolerated."

In the nearby province of Aleppo, rebels and troops fought fierce battles around the military air base of Kweiras, which opposition forces have been trying to capture for weeks, the Observatory said.

In the city of Aleppo, Syria's largest urban centre and commercial capital, rebels and troops battled around the international airport and the nearby air base of Nairab, said the Observatory and the Aleppo Media Center.

Fighting also raged in parts of Damascus, where troops bombarded rebel-held southern neighbourhoods, according to the Observatory. It said at least one person was killed in the Palestinian refugee camp of Yarmouk by sniper fire.

In the southern region of Quneitra rebels briefly controlled a military checkpoint in the town of Khan Arnaba and captured five officers before they withdrew, the Observatory said. It added that the rebels captured a tank then blew it up.

Quneitra is on the cease-fire line between Syria and Israel, which controls most of the Golan Heights after capturing the strategic territory from Syria in the 1967 war.