Before the war officials estimated some 2,000 such deadly missiles were circulating out of the control of national armies. Now they believe the number is around around 6,000, according to the Times.

Shoulder-fired missiles are small and easy to use against aircraft.

US analysts estimated that Saddam's regime had a stockpile of at least 5,000 shoulder-fired missiles, but less than a third have been accounted for, leaving some 4,000 missiles missing, according to the Times.

The missing Iraqi missiles include Russian-built SAM 7 and SAM 16.

Officials do not know if the missiles were removed during the US-led invasion or looted from arsenals after the regime collapsed.

It is also unknown if the missiles have been taken out of the country or are in the hands of Iraqi insurgents.

Since the fall of Saddam's regime about a dozen shoulder-fired missiles have been used to attack US planes or helicopters. US officials say there have been few recent attacks because of precautionary tactical measures.