Syrian military helicopters systematically dumped canisters of chlorine gas, a banned weapon, on residential areas of Aleppo at least eight times late last year in the final weeks of the battle to retake the city from rebels, Human Rights Watch said in a detailed study released Monday.

The assertions in the study, if confirmed, would represent one of the most egregious uses of such outlawed weaponry in the war. It would amount to a new level of impunity by the Syrian president, Bashar al-Assad, who promised to never use chemical arms under an international treaty the government signed more than three years ago.

A United Nations panel that has been investigating reports of chlorine bombs and other chemical weapons in the Syrian war concluded last year that government forces had used them at least three times in 2014 and 2015. The panel is scheduled to provide an update this month.

Mr. Assad and his subordinates have repeatedly denied that their side has ever used chemical weapons, calling the evidence fabricated or inconclusive.