The jihadi group surging through Iraq and Syria is using large captured US-made weapons and has access to anti-tank rockets supplied by Saudi Arabia to a moderate rebel group, according to a report published on Monday.
The study by the London-based Conflict Armament Research consultancy found that Islamic State (Isis) militants had access to large numbers of US weapons, which they were shifting to key battlefields.
The report drew no conclusions about how the weapons were sourced. However, the capitulation of the Iraqi army in northern Iraq on 10 June gave the jihadis access to military arsenals in the north of the country, which were full of US-supplied assault rifles and ammunition, as well as heavy weapons.
The report was compiled from a list of weapons captured from Isis by Kurdish militias over a 10-day period in July.
Of most interest was the capture of two M-79 rockets that were identical to a batch of such weapons supplied by Saudi Arabia to rebels in southern Syria in January 2013. The rockets had been sourced from a Croatian arms supplier and ferried to anti-Assad fighters who were identified as non-jihadis.
The potential for weapons provided to one group to fall into the hands of other militants fighting in Syria has often been cited by the US and others as a reason not to heavily arm groups involved in the civil war.
Calls by rebels for heavy weapons that would allow them to confront Syria's air force or tanks have regularly been turned down. But with the war zone now flush with such weapons – largely in the hands of jihadis who oppose both mainstream rebels and the Syrian regime, Washington is rethinking its involvement.
Moderate rebels in northern Syria have in the past fortnight confronted Isis forces who arrived in Humvees and armoured troop carriers supplied by the US to Iraq. The rival rebel groups found it difficult to stop Isis with the weapons they had.
The vehicles were seized en masse from bases near Mosul and driven across the Syrian border, where they have revitalised a battlefield that had become a three-way stalemate in eastern and northern Syria.
US jets have in the past six weeks bombed Humvees and US-made troop carriers in northern Iraq. Many were used by the US military during the nearly nine-year war and occupation, then handed over by US officers as they left Iraq. Others were bought directly from Washington.
Using such vehicles, Isis fended off assaults on Sunday in far the western Anbar province by Iraqi forces, backed by fighter jets. US jets again attacked Isis positions near the Haditha dam, the second one of Iraq's two most important waterways, which has become a key target for jihadis.