MILITANT insurgent groups responsible for killing thousands of Iraqi civilians over the past two years are guilty of war crimes and crimes against humanity, Human Rights Watch charges today.

In a report chronicling the killing of an estimated 15,000 men, women and children by insurgents, the organisation insists that international law applies equally to US-led forces as well as those claiming to resist foreign occupation. “The armed conflict in Iraq is regulated by the 1949 Geneva Conventions and customary international laws,” says the report, Iraq: Insurgent Groups Responsible for War Crimes.

Many insurgent groups are operating in Iraq, from Islamic extremists to members of the ousted Baath party and Sunni Muslim nationalists. Three groups are blamed for most of the worst abuses: al-Qaeda in Iraq, headed by Abu Musab al-Zarqawi, Ansar al-Sunna (Supporters of the Sunni) and Ansar al-Islam (Supporters of Islam).

Human Rights Watch has produced numerous reports critical of the US military and the Iraqi Government’s behaviour since the invasion in 2003. Sarah Leah Whitson, who heads the group’s Middle East and Africa division, said it was important that insurgents were also called to account.

The report coincides with fears that foreign fighters in Iraq, responsible for some of the worst outrages, may return home and use similar tactics against pro-Western governments in the Arab world and beyond.

The claim was based on captured documents found with Abu Azzam, a commander in al-Zarqawi’s group, who was killed last month in Baghdad. “We got hold of a very important letter from Abu Azzam to Zarqawi asking him to begin to move a number of Arab fighters to the countries they came from to transfer their experience in car bombings in Iraq,” Bayan Jabor, the Iraqi Interior Minister, said. A report by the Centre for Strategic and International Studies said that there were about 30,000 insurgent fighters in Iraq, with foreign volunteers making up to 10 per cent.

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