A Briton has been arrested in the US after a suspected plot to supply a lethal surface-to-air missile to terrorists.
It is thought the alleged target might have been a commercial aircraft or the US President's personal aircraft.
Western intelligence officials have confirmed to the BBC it was a multinational sting operation, involving agents from the US, Russia and Britain.
It resulted in the arrest of a British arms dealer at Newark in New Jersey on Tuesday.
Two people were later arrested in New York and the trio are expected in court on Wednesday.
Officials say the British man successfully imported a Russian-made Igla missile into the US and believed he was selling it to an Muslim extremist.
But his buyer was an undercover FBI agent and the arms dealer's voice is heard on tape saying he wanted the missile to be used to shoot down a large passenger plane.
The FBI have denied that the plane in question was Air Force One, the US President's official aircraft.
Russian-made surface-to-air missile
believed to have been responsible for the shooting down last year in Chechnya of a Russian troop-carrying helicopter
They insist they were aware the missile was being brought into the US at Baltimore docks, shipped from Russia and disguised as medical equipment.
Although no actual terrorists are thought to have been involved in the operation, intelligence officials said it was a terrifying illustration of the vulnerability of Western nations to attack by extremists.
Over the last 15 months, there have been three foiled attempts by groups linked to al-Qaeda to shoot down planes carrying Western or Israeli passengers.
BBC correspondent Tom Mangold said the man bought one for $85,000 by corrupt middle management at a Russian factory and was promised another 50.
He said: "It may not have happened, but when the dealer says he wants to bring down Airforce One, it's pretty dispiriting."
The FBI deny the presidential aircraft was a target
He said apartments in London were being searched by Scotland Yard and more arrests could follow.
The sting demonstrated "fantastic" cooperation between the US, Russia and Britain, added Mangold.
"Old enemies really have joined hands," he said.
MI5 and MI6 in London were involved with Russian
secret services FSB and the FBI.
Russia detected the arms dealer five months ago in St Petersburg and Moscow, then kept under surveillance.
Russian President, Vladimir Putin, authorised for the FBI to have an undercover agent sent to Russia to work with the FSB.
The arms dealer flew to New York with his wife on Sunday at 1330 BST on a British Airways flight from Heathrow Airport in London.
But he was followed on to the plane by an FBI agent and arrested in New Jersey after he collected a package marked 'medical supplies'.
The man is an established arms dealer, thought to be a middle aged man of Indian origin, who lives in London.
Washington officials are remaining tight-lipped about the operation but admitted there had been an arrest.