The US is paying two mine-clearing groups from Britain and Switzerland nearly £620,000 to find anti-aircraft missiles in Libya that could end up in the hands of terror groups.

The Gaddafi regime's military amassed nearly 20,000 of the weapons before the popular uprising began in March.

The US State Department's hiring of the UK's Mines Advisory Group and the Swiss Foundation for Mine Action was prompted by fears that terrorists could use scavenged man portable air defence systems, known as MANPADS.

The move has no effect on the massive numbers of mostly Russian-built anti-aircraft launchers and missiles still in the hands of Muammar Gaddafi's forces.

"I can't imagine the US can do anything about Gaddafi's inventory until they defeat him or negotiate his exit," said Matthew Schroeder, an arms expert with the Federation of American Scientists in Washington.

The Obama administration listed the anti-MANPADS effort in a report to Congress defending the legality of its intervention in Libya.

The report included classified documents detailing a "threat assessment of MANPADS, ballistic missiles and chemical weapons in Libya."

Officials with the two weapons demolition teams hired by the State Department said almost all of the Libyan weapons depots they surveyed in recent weeks showed clear signs of looting.

The Mines Advisory Group located and destroyed two of the portable missile systems near the north-eastern Libyan opposition-controlled town of Ajdabiya last week, spokeswoman Kate Wiggans said.

The group also found two other stray anti-aircraft missiles in May and destroyed them. All four were SA-7s, Russian-made portable missiles that date to the 1970s.