BEIRUT, Lebanon — Syrian antigovernment activist groups on Monday reported what appeared to be a major escalation of the military’s prolonged siege of Aleppo, with video and witness accounts of a helicopter gunship attack in which crews dropped explosive barrels filled with TNT and shrapnel on more than 13 rebel-held neighborhoods.

Reports by the Syrian Observatory for Human Rights and others said that between 76 and 118 civilians, including as many as 28 children, were killed in the air assault on Sunday. The scope of the destruction appeared to be enormous, based on video clips uploaded by the observatory and Syrian social media users that could not be immediately corroborated for their authenticity.

The observatory, a group based in Britain that monitors the Syrian war through a network of contacts on the ground and has reported on brutalities by both sides, reported further air attacks on Monday, both in Aleppo and near the southern border with Jordan. But the extent of casualties and damage was unclear.

Improvised barrel bombs, which contain hundreds of pounds of explosives and shrapnel that includes metal shards and iron nails, have been used before by the Syrian Air Force in the 33-month conflict and have been condemned by rights groups as a particularly insidious weapon that kills indiscriminately.