PAUSE for a moment and consider this terrifying scenario. An airliner is making its final approach to land at an international airport. Well outside the airport perimeter far from security checks a terrorist lies in wait. As the plane drops below 15,000 feet, the terrorist takes from his car boot or a large holdall, a light, portable, shoulder-held missile launcher, fires it and brings down the aircraft killing all on board.

It might seem only like the stuff of movie thrillers or spy novels, but it’s precisely such a potential attack that has haunted counter-terrorism officers across the world for years.

To carry out such a strike any terrorist would need what’s known by its acronym as a ManPad, short for “man-portable air defence system”.

For decades now more than a million ManPads have been manufactured worldwide, with as many as 20 countries having produced or having licences to produce the weapons or their components. The US, Russia, China and Britain are among them. Cheap, portable, easy to conceal, use, and highly lethal, and these days increasingly available on the arms black market, they are known as a “fire and forget” weapon. In other words, once the missile is fired it locks on to the heat source of an aircraft like the exhaust, and flies right up the tailpipe and explodes.

Such is the threat they pose should they fall into the hands of terrorists that after the toppling of Libyan dictator Muammar Gaddafi in 2011 the CIA is said to have launched a concerted effort to try and track down and recover ManPads looted from Libyan stockpiles by opposition rebels.

Not, though, before though some of the weapons had found their way from Libya to Syria via Turkey. For the inescapable fact is that the CIA has not always been at the recovery end of things. The agency itself is known to have supplied various rebel groups with the weapons in the past, most notably Afghan guerrillas fighting the Soviets in the 1980s. Which brings me to an event that occurred only last week in Syria.

So remote, complicated and seemingly interminable is the war there that it’s hardly surprising most of us pay little attention to the bloody events that play out daily. Last Saturday though, the shooting down of a Russian Su-25 warplane marked yet another dangerous sign of the ways in which the war in Syria has the potential to come back and impact on any of our lives. I’m speaking of course about terrorism.

That the plane was shot down using a ManPad now seems certain. That

an al-Qaeda linked group Tharir al-Sham also claimed responsibility, saying one of its fighters scored a direct hit with a shoulder-launched anti-aircraft missile, has once again set alarm bells ringing in some counter-terrorism circles.

The inevitable blame game has got under way, with Russia suggesting the Americans are now supplying ManPads to Syrian jihadist rebels, while Washington denies any such thing.

The most important thing to remember from the outset here is that no one is squeaky clean when it comes to weapons supplies in Syria. Weapons for the war come from myriad sources, some overtly supplied and delivered, some covertly. For most of the major powers now embroiled in Syria, though, the risk of what intelligence agencies call “blowback” from terrorism carried out by weapons they supply has always been a gamble. In the morass of the Syrian conflict, determining just who the "good" and "bad" guys are, and who exactly constitutes a safe bet when it comes to giving them weapons, has always been a lottery.

Few weapons though carry the risk of such devastating blowback more than ManPads. As last Saturday’s shooting down of the Russian warplane reminded us, terrorist groups and those affiliated with them increasingly find it easy to acquire them.

In the shadowy world of Syrian factional intrigues, where al-Qaeda-aligned militias collaborate with openly pro-Western groups the CIA and US Special Forces, it’s near impossible to know exactly who supplies ManPads or gives the orders to fire as happened against the Russian plane.

The proliferation of and accessibility to such weapons however should be a wake up call for all. From grey and black market sources, arms dealers, front companies, trans shipment, intermediaries, end-use certificate falsification, and corrupt government officials, terrorists are demonstrating increasingly sophisticated and aggressive approaches towards acquiring these weapons that could so easily be used to devastating effect against civilian airliners.

That governments including those in the West are murkily involved in such a process makes a mockery of their claims to be fighting the good fight in the “war against terror.”

With such weapons already in the hands of jihadist groups, just how long will it be before use of a ManPad moves from the battlefields of Syria to be used in a terrorist "spectacular elsewhere?

That the use of ManPads in terrorist attacks has not been more frequent is perhaps testimony to the efforts of intelligence services to counter their threat.

Since 1975 about 40 civilian airliners have been targeted by ManPads, but it was the attempted shooting-down in 2002 of an Israeli passenger jet taking off from Mombasa, Kenya, that really focussed some minds on countering their threat.

There seems little point though in adopting such a strategy while those same weapons are, through other quarters, being covertly passed to those who in turn might hand them over wilfully or through capture to terrorists.

Such is the existing double standards that US officials have even talked of designing ManPads that include components that make them inert after a period of time or the user having to insert a code before use.

One twist in the concept involves modifying batteries in such a way that they no longer function after a certain period. But as Syrian rebels highlighted some time ago after posting videos on YouTube, they then found ways to fire ManPads using improvised batteries.

Surely rather than trying to modifying the weapons with the notion of making them terrorist proof the real aim should be to stop them getting into their hands in the first place.

Quite clearly new multilateral efforts must now be made to hold governments as well as black market arms dealers to account for the supply of such weapons. That the inevitable blowback is coming at the hands of Islamist-inspired terrorists is almost certain without it.