The Syrian regime was accused Sunday of dropping cluster bombs — indiscriminate scattershot munitions banned by most nations — in a new sign of desperation and disregard for its own people.

The international group Human Rights Watch cited amateur video and testimony from the front lines in making the allegation against the government of President Bashar al-Assad.

Syria and Turkey, meanwhile, declared their skies off-limits to each other amid mounting cross-border tensions. Turkey is an outspoken supporter of rebels trying to oust Assad in a conflict that has raged for about 19 months and has turned into a civil war.

The ban on overflights is part of an increasingly assertive Turkish stance toward Syria that has stirred concerns about a regional conflagration. In the past two weeks, Turkey has retaliated for stray Syrian shells and mortar rounds, intercepted a Syrian passenger plane on suspicion that it carried military equipment, and — according to a Turkish newspaper Sunday — sent more warships to naval bases north of the Syrian coastline.

Inside Syria, rebel fighters and regime forces have been locked in a bloody stalemate for weeks, with rebels holding large rural stretches in the heavily populated west but unable to dislodge troops from urban centers. During the summer, the regime escalated shelling and airstrikes on rebel-held areas.

Interactive: Recent events in Syria

Human Rights Watch said new amateur videos and interviews with residents suggest that the Syrian air force has dropped cluster bombs in the past week, primarily along a main north-south highway in western Syria that runs through Maarat al-Numan, a town captured by rebels after fierce fighting.

Cluster bombs open in flight, scattering smaller bomblets over a wide area. Many of the bomblets do not explode immediately, posing a threat to civilians long afterward.

Steve Goose, an arms expert for the New York-based human rights group, said that most nations have banned cluster bombs and that many of those that have not, including the United States, have said they would do so soon.

“These are weapons that are really beyond the pale,” Goose said in a phone interview. “This is a weapon of desperation [for Syria] at this point in time. Only those governments and political leaders who are willing to thumb their nose at international opinion will use these weapons.”

The Syrian government had no immediate comment.

The first word of cluster bombs being dropped by the Assad regime emerged in July, but the recent reports indicated a more widespread use, said Nadim Houry, a Lebanon-based researcher for Human Rights Watch.

Sunday’s report said activists posted at least 18 videos in the past week showing remnants of the bombs in or near the central city of Homs, the northern cities of Idlib and Aleppo, rural areas near the town of Latakia and the eastern Ghouta district close to the capital, Damascus. The group also spoke to residents in Taftanaz and Tamane who said cluster bombs were dropped in their towns Tuesday.

There was no report of casualties from the recent cluster bombs, the report said, adding that the munitions shown in the videos were made in the Soviet Union, a major arms supplier to Syria before the bloc’s collapse in 1991.

— Associated Press