Eyewitness accounts and evidence collected from northwestern Syria "strongly" suggest regime forces dropped toxic chemicals on civilians several times last month, Human Rights Watch said on Tuesday.

The New York-based rights group said the chemicals appeared to have been packed into crude explosives-packed barrels that were dropped by military helicopter on rebel-held areas during heavy fighting for the city of Idlib.

"Evidence strongly suggests that Syrian government forces used toxic chemicals in several barrel bomb attacks in Idlib governorate between March 16 and 31, 2015," HRW said.

It called on the UN Security Council to investigate what would be a breach of both its own resolutions and Damascus's obligations under the Chemical Weapons Convention.

HRW said it had investigated six reported attacks in Idlib and villages outside, collecting evidence from rescue workers and other civilians that provided a compelling case in three of them.

The most conclusive evidence came from a March 16 attack on the village of Sarmin, which left a family of six, including three children, dead, and an attack on Idlib city on March 31.

"The children were foaming at the mouth, they were suffocating, then their hearts stopped," said Leith Fares, a rescue worker in Sarmin.

HRW said it could not conclusively establish the chemical used but volunteers from the Syrian Civil Defence said they found remnants of barrel bombs at attack sites and smelled chlorine gas on victims' clothes.

The world's chemical weapons watchdog already expressed "serious concern" on March 25 over the reported use of toxic agents in Idlib province.

In January, it reported the use of chlorine gas in three attacks on three Syrian villages last year.

The Organization for the Prohibition of Chemical Weapons did not attribute responsibility for those attacks, although its report cited witnesses saying they heard helicopters, which only the regime possesses.

In early March, the UN Security Council adopted a US-drafted resolution condemning the use of chlorine in Syria and threatening sanctions if the chemicals were used again.

"The Syrian government appears to be thumbing its nose at the Security Council and international law yet again," HRW's deputy Middle East director Nadim Houry said.

Damascus did not have to declare its stocks of chlorine under a 2013 agreement to dismantle its chemical arsenal as it is widely used for commercial and domestic purposes.

But use of the gas for military purposes would be a breach of its undertakings under the Chemical Weapons Convention, which it signed as part of the deal.