The Russians supplied a deactivated missile for the FBI sting, but half a million others are on the world market. Attempts to use them to shoot down commercial jetliners are the source of growing fears in the US administration.

The United States is itself partly responsible for the number of missiles on the international black market.

Stinger missiles supplied by the United States to opposition groups during the Soviet occupation of Afghanistan in the 1980s and the Strelamissile used by the Russian army in Chechnya are the most popular models.

Both can hit a helicopter or a slow-moving plane at low altitude.

More recent missiles -- such as the SA-18 that Hemant Lakhani was allegedly trying to smuggle into the United States when detained on Tuesday -- have a range of 3,500 metres (11,400 feet).

According to a recent Congress report there were between 500,000 and 700,000 shoulder fired surface-to-air missiles of various kinds in circulation at the start of the year.

Experts say that any commercial jet could now be a target as it takes off or lands at an airport. Some planes have already had near misses.

US authorities began to worry after two SA-7 missiles narrowly missed an Israeli Boeing 757 charter flight taking off from the Kenyan resort of Mombasa in November with 261 passengers. Al-Qaeda, which staged the September 11 attacks in New York and Washington in 2001, has been blamed for the attempt.

The registration numbers of these missiles were close to those of missiles fired at -- and narrowly missing -- a US military transporter taking off from Prince Sultan Air Base in Saudi Arabia in May, according to intelligence sources quoted Wednesday by The New York Times and The Washington Times.

The Washington daily quoted Israeli intelligence saying that missiles from the same batch were also found near Prague's airport in 2002 after a failed attempt to bring down an El Al airliner carrying Israeli Foreign Minister Shimon Peres on a visit.

The US government has sent air security experts to Iraq, Europe and Asia to assess the threat to US jets at vulnerable airports.

But the White House opposes a proposed law currently blocked in Congress which would force the government to put anti-missile equipment in all US-registered commercial jets.

James Carafano, a retired colonel and now a senior research fellow on defense and homeland security at The Heritage Foundation, said the price had been "inflated" by critics.

"The number that was kicked around was between one and three million dollars per aircraft. I believe the number is much less than that," he said.

"This is not new technology, the systems you require don't have to be so sophisticated as for a fighter jet.

"We are talking about a few missiles being fired by unexperienced users."

Several members of Congress have used Tuesday's arrests however to reinforce their campaign to get the legislations passed.

"People do overstate the threat in the sense that it's not exactly easy," added Carafano.

"Most of these Russian weapons only have a maximum range between an altitude of 3,500 and 4,000 meters. It means you can knock down a commercial airliner only when it's taking off or landing, it's in the first or last five to 10 minutes of flight that the plane is at risk.

He said: "Even for a trained guy, you have about a 50-50 shot to hit the plane and if you are an untrained guy, it's much less than that."

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