Brian Roehrkasse, a Department of Homeland Security spokesman, said US experts have been sent to help improve security in Basra and Baghdad in Iraq, as well as in a number of Asian and European airports, which he declined to specify.

Investigators from the federal Transportation Security Adminstration, are working with the Federal Bureau of Investigation, the Department of State and other US agencies in the effort to improve security at the airports, he said.

"We are going to countries that want to work with us," Roehrkasse said.

"They are airports that have US-flagged aircraft flying in and out of them."

Similar efforts to improve airport security from portable missile attacks are already underway at US airports, Roehrkasse said.

The move is part of the goverment's response to recent intelligence reports suggesting that a terrorist attack using small heat-seeking missiles may be imminent.

The United States has drastically tightened security at domestic airports since the September 11 attacks in 2001 when jets hijacked by al-Qaeda terror group members slammed into the World Trade Center towers in New York and the Defense Department headquarters in Washington.

Roehrkasse said that while "the US intelligence community does not have any specific intelligence that al-Qaeda intends to use these weapons for a major attack against US commercial aviation," officials felt concerned there is a growing possibility of such an attack.

The best-known portable missiles are the American-made Stinger and the Russian-made SA-7. The Stinger was once widely used by Afghan groups close to al-Qaeda.

Aviation officials have said the missiles, which can be bought on the black market for as little as 5,000 dollars apiece, can hit jets from as far as 30 miles (50 kilometers) from an airport and can reach altitudes of about 10,000 feet (3,000 meters) for older systems and 18,000 feet (5500 meters) feet for newer weapons.

Officials began to focus on the threat posed by terrorists with shoulder-mounted missiles last November, when unknown assailants narrowly missed an Israeli charter flight taking off from Mombasa, Kenya.

In May, another missile missed a US military jet taking off from Prince Sultan Air Base in Saudi Arabia. Al-Qaeda is suspected in both attacks.

In Iraq last month, a US C-130 military transport plane came under fire from a surface-to-air missile as it landed at Baghdad's airport.

That attack came as US forces noted an increase in the "sophistication" of guerrilla strikes in Iraq, where dozens of US troops have been killed since May 1, when US President George W. Bush declared the end of major combat operations.

A US congressional report this year determined that the worldwide inventory of portable surface-to-air missiles probably exceeds 500,000 and may be as high as 700,000.

US lawmaker John Mica, chairman of the House of Representatives' Aviation Subcommittee, which issued the report, has introduced legislation calling for outfitting all of the nearly 7,000 US commercial aircraft with anti-missile technology. The move would cost up to two million dollars per plane.

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