Five Syrian rebel groups have been accused of a "chilling" wave of torture, abductions and summary killings in the northern provinces of Idlib and parts of Aleppo.

Amnesty International, a UK-based rights group, said on Tuesday that al-Nusra Front, Ahrar al-Sham, Nureddin Zinki, the Levant Front and Division 16 were committing "war crimes and other violations of international humanitarian law with impunity.

"Many civilians live in constant fear of being abducted if they criticise the conduct of armed groups in power or fail to abide by the strict rules that some have imposed," said Philip Luther, Amnesty's director of the Middle East and North Africa Programme.

The report was based on interviews with about 70 individuals living or working in the northern provinces of Idlib and parts of Aleppo, areas controlled by rebels.

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The abuses were committed over four years by five armed groups, including some backed by the US and other regional powers, Amnesty said.

"While some civilians in areas controlled by armed opposition groups may at first have welcomed an escape from brutal Syrian government rule, hopes that these armed groups would respect rights have faded as they have increasingly taken the law into their own hands and committed serious abuses," said Luther.

'Cease arms transfers'

The report documented at least 24 abductions of activists, ethnic and religious minorities, as well as three children, two of whom remain missing as of last week.

Amnesty also documented summary killings by gunfire, some in public, of pro-government fighters.

It called on international backers to cease arms transfers to groups implicated in the abuse.

One of the groups, Ahrar al-Sham, said in a letter that it would like to meet Amnesty to clarify the issues. It did not respond to the allegations.

Syria's conflict began in March 2011 with anti-government protests but has since broken out into all-out war, leaving more than 250,000 people dead.

Source: Al Jazeera and agencies