Syrian activists say a string of government airstrikes on rebel areas in the country's north has killed at least 20 people, leveled buildings and forced residents to dig through mounds of rubble in search of survivors.

The activists say the strikes happened late Wednesday and early Thursday and hit a total of four towns in Idlib and Aleppo provinces.

They say residents are searching through the rubble in some of the towns, while in others the dead were buried before being identified.

Videos of the strikes posted online show leveled buildings and survivors pulling bodies from the debris.

Activist claims and videos could not be independently verified.

President Bashar Assad's forces have increasingly relied on air power as the rebels have improved their fighting capabilities on the ground.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

BEIRUT (AP) _ Syria's state news agency says rebels have blown up two oil and gas pipelines in the northeast near the Iraqi border.

The agency, SANA, says the attack hit one oil and one gas pipeline Thursday near the city of Deir al-Zour. The pipelines ran between Deir al-Zour and the city of Palmyra in central Syria.

Anti-regime activists posted a video online purporting to show the blast site, with thick smoke billowing into the sky. The video could not be independently verified.

SANA quoted an oil ministry official saying the lines were immediately shut off, the fires were extinguished and repairs would begin soon.

Rebels seeking to overthrow President Bashar Assad have repeatedly bombed such pipelines.

Activists say more than 33,000 people have been killed in 19 months of violence.