An alleged chlorine gas attack by the Syrian government in a rebel-held town is evidence of new war crimes, right body Amnesty International has said.

The London-based human rights organisation said on Tuesday that the alleged chlorine gas attack in Idlib that activists said killed an entire family, is evidence of more war crimes by the Syrian government.

"These horrific attacks that resulted in civilians, including small children, suffering excruciating deaths, are yet more evidence that the Syrian government forces are committing war crimes with impunity," said Philip Luther, Middle East and North Africa Programme Director at Amnesty International.

"The situation in Syria must be referred to the International Criminal Court as a matter of urgency."

Three children, their mother and father, and their grandmother suffocated to death after the barrel bomb attack in the town of Sarmin in southeast Idlib province, the Syrian Observatory for Human Rights said.

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The Britain-based monitoring group told AFP news agency that doctors in Sarmin concluded that the manner of death indicated a gas, possibly chlorine, had been emitted from the barrel bombs.

A Syrian military spokesperson has denied responsibility for the attacks.

'Ruthless airstrikes'

Amnesty also said in another report on Tuesday that the Syrian government's attack on a bastion of the Islamic State of Iraq and the Levant (ISIL) group in 2014 killed 100 civilians and carries "every indication of being war crimes".

Amnesty defined the attack launched by the government in November 2014 on the city of Raqqa as a series of "ruthless air strikes".

It said the strikes carried out between November 11 and 29 killed 115 civilians, including 14 children, and hit non-military targets including a mosque, a transport hub and a busy market.

Raqqa is the self-proclaimed capital of ISIL, but Amnesty said there was no indication that fighter positions were the target of the series of strikes.

"Syrian government forces have shown flagrant disregard for the rules of war in these ruthless air strikes," Amnesty's Luther said.

"The government appears indifferent to the carnage caused by these strikes, refusing even to acknowledge civilian casualties they have caused."

Luther acknowledged that Raqqa is a stronghold of ISIL members, but said their presence did not justify attacks on civilian targets.

US drone shot down

It urged an arms embargo to "stem the flow of weapons being used to commit these crimes".

ISIL controls chunks of territory in Syria and Iraq where it has declared an Islamic caliphate.

For its part, Syria's state news agency said the country's air defence forces shot down a US reconnaissance aircraft.

"Syrian air defences brought down a hostile US surveillance aircraft over north Latakia," the Syrian state news agency SANA reported.

The US military confirmed that it had lost communication with an unarmed Predator drone over northwest Syria on Tuesday and was looking into the claims it was brought down.

If confirmed, it would be the first time that Syrian forces have attacked a US aircraft since the coalition fighting the IISIL began raids against the armed group in Syria in September.