By Paul Reynolds BBC News Online world affairs correspondent

In the old days of the Cold War, the main task of the UK internal security service MI5 was to monitor the activities of KGB agents. From time to time, large numbers of them were thrown out.

The Igla has a 4km range and an infra-red facility

These days the FSB, the KGB's successor in counter-intelligence and internal security, has its own liaison officer to the British intelligence agencies in London. It also has one in Washington.

The cooperation between the FSB (the Federalnaya Sluzhba Bezopasnosti, or Federal Security Service) and the British and Americans in the case of the Russian anti-aircraft missile is a product of this new world order - and of fierce self-interest on all sides.

The American and British governments decided long ago that they needed Russian help in the war on terror declared by President Bush after 11 September.

The FSB regards Islamic terrorism as a common enemy and is only too happy to help Mark Galeotti Russian specialist

The deal was that they would turn a blind eye to Russian activities in Chechnya. They have kept their side of the bargain.

President Bush cemented this arrangement in a personal relationship with Russian President Vladimir Putin, himself the former head of the FSB. British Prime Minister Tony Blair has trod a similar path.

The Russians have been very keen to help. And this case shows that they are bringing something to the table.

Russian PR

According to Mark Galeotti, a Keele University specialist in Russian organised crime and a writer for Jane's Intelligence Review, the FSB has several motives.

"I am not surprised at the level of cooperation," he said.

The US slapped sanctions on a Chechen rebel leader last week

"The FSB regards Islamic terrorism as a common enemy and is only too happy to help. It wants to paint all Chechen rebels as Osama bin Laden terrorists and argues that this is therefore a common fight.

"It would also have been catastrophic for Russian relations with the United States had this dealer managed to get hold of a Russian missile and a plane - say, over Seattle - was shot down with it."

Mark Galeotti said that the FSB had another reason to be cooperative:

"The FSB is very active but not very good. It is still really much like the old KGB and it spends a lot of time intimidating likely dissidents.

"It has failed to predict the actions of Chechen rebels [as was evident in the attack on the Moscow theatre last year] as it has few agents in place and relies too much on intercepting communications."

"It badly needed a success."

He was not surprised that the FSB had picked up on the arms dealer wanting an anti-aircraft missile:

"A Russian taxi driver could get you a crate or two of Kalashnikovs but a request for missiles would be unusual."

Putin's role

The dynamism behind this approach is Mr Putin.

An ex-KGB officer himself, he headed the FSB until appointed Acting Prime Minister by Boris Yeltsin in 1999. One of Mr Putin's former KGB colleagues Nikolai Patrushev is now head of the FSB.

Mr Putin listens to the FSB and, according to Mark Galeotti, listens to little else.

"The FSB is Putin's right hand," he says. "It briefs him and not only on intelligence. He listens to its reports on the economy as well. He turns to the FSB if he wants a problem solved. It has the size and scope to let it act fast. Putin too wants the FSB to be successful."

There have been other examples of Russian co-operation with the West over intelligence.

The Russians helped provide information on Afghanistan before and during the war against the Taleban.

They have also helped in countering organised crime and in tracking money-laundering.

But all this has not stopped other Russian intelligence agencies from pursuing economic and military information in the West in many of the same old ways. Self-interest prevails, of course, on all sides.

New co-operation

This case has shown that there is a need for intelligence-sharing. Al Qaeda is presumably still interested in further spectacular aviation attacks.

An attempt was made in November 2002 to shoot down an Israeli plane over Kenya with two shoulder-held anti-aircraft missiles known as SA-7s - the forerunner of the Igla missile in the present case.

Thousands of these weapons are in military use around the world.

If someone was looking for one, I would tell them to go to Chechnya and find some disgruntled Russian troops Jim O'Halloran, Jane's defence editor

Such is the concern about them that the G8 industrial countries - that is, the West and Russia - actually mentioned missiles when they committed themselves to action at their meeting this year:

"We commit ourselves to preventing terrorists from acquiring nuclear, chemical, radiological and biological weapons; missiles and related materials."

Jim O'Halloran, editor of Jane's Land-based Air Defence, says that the Russians are important in helping prevent missiles from getting into terrorist hands:

"If someone was looking for one, I would tell them to go to Chechnya and find some disgruntled Russian troops. The rebels have them as they have shot down Russian helicopters with them.

"The SA-7 is the most widely deployed system in the world. Everyone has them.

"What is worrying about the present case, though, is that the Igla is much more expensive.

"For a couple of thousand dollars you could cause chaos with an SA-7 even if you did not fire it. But to spend $85,000 on an Igla as this dealer did, you have to have a high-value target in mind."

Plane safety

Threats to American and British aircraft are constant. British Airways cancelled flights to and from Kenya for a period and have now done the same in Saudi Arabia.

American officials have recently been inspecting airports worldwide in case they are especially vulnerable to anti-aircraft fire and what, if anything, could be done about it.

The Department of Homeland Security has ordered close searches of electronic equipment being taken on board by passengers.

Senator Barbara Boxer from California has introduced a bill to equip all commercial aircraft with anti-missile technology as used by the military. This would cost an estimated million dollars per plane.

Following the arrests in the case, she said: