US, Saudi-bought arms from EU fell into IS hands: monitor

"The United States and Saudi Arabia supplied most of this materiel without authorisation, apparently to Syrian opposition forces," said Conflict Armament Research in a report seen Thursday.

CAR, which monitors the movement of arms into conflict zones, said Washington and Riyadh, supporters of rebel groups in the Syrian conflict, had not secured the required authorisation to transfer the purchased weapons to another end user.

"This diverted materiel, recovered from IS forces, comprises exclusively Warsaw Pact-calibre weapons and ammunition, purchased by the United States and Saudi Arabia from European Union (EU) member states in eastern Europe," it said.

The weapons were bought from countries including Bulgaria and Romania, according to the report.

But it said the United States "did not notify the supplier states concerned before retransferring the materiel".

"Although these proportions are low, IS forces captured significant quantities of NATO weaponry during initial assaults on Iraqi forces in 2014."

CAR said that arms supplied to combatants in Syria by foreign parties, notably the United States and Saudi Arabia, had "indirectly allowed IS to obtain substantial quantities of anti-armour ammunition".