FOR counter-terrorism officers it is the stuff of nightmares. A terrorist lies in wait as an airliner drops below 15,000 feet and begins its final approach to land at an international airport.

Well outside the airport perimeter, far from security checks, the terrorist takes from his car boot or large holdall a light, portable, shoulder-held missile launcher, fires it and brings down the aircraft killing all on board.

The weapon used is known by its acronym, Manpad, short for “man-portable air defence system”.

Cheap, 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 or engine, and flies right up and explodes.

Two weeks ago the devastating use of a Manpad became apparent when a Russian Su-25 warplane was brought down in the rebel-controlled province of Idlib in north-western Syria. And yesterday morning an Israeli F-16 warplane was brought down over Syria, apparently by a ground-to-air missile, although it is now yet known if it was a Manpad.

For some time now Manpads have been making their mark in the Syrian conflict. The weapons system has proliferated across the country due to a combination of seizures from overrun and captured government stockpiles and the supply of outside actors determined to erode Syrian, or Russian, air power in the war there.

Against the murky backdrop of arms shipments and supplies to Syria, the sources from which jihadist fighters procure Manpads remains difficult to definitively ascertain. Russian officials left no doubt that Moscow aimed to establish precisely where the weapon used against its aircraft came from and openly pointed the finger at the United States.

Russian Senator Frants Klintsevich, deputy chairman of the Russian Federation Council Defence and Security Committee, said, “I am absolutely convinced... that today the militants have Manpads, and they were supplied by the Americans through third countries,”

The Russians have a long memory of US supplies of these weapons to their enemies.

In the 1980s, in Afghanistan, Russian aircraft, including Su-25 jets, were frequently attacked and often hit by US Stinger missiles, supplied to Afghan mujahideen fighters by the CIA.

But, unlike in the 1980s, the aircraft shot down in Syria was no Afghan War relic, but a modernised warplane with an automated onboard missile-defence system that should have deflected a Manpad strike. This would suggest that the defence system either malfunctioned or the missile used by the jihadists to down the jet was too modern and sophisticated.

For its part the Pentagon responded to the downing of the Russian aircraft by quickly denying it supplies such weapons to US-backed militias in Syria.

But the denial belies some uncomfortable truths. As far back as December 2016 US President Barack Obama, who was sharply critical of Russia’s intervention in Syria, signed the annual defence policy bill into law, a move many – especially Moscow – saw as easing restrictions on arming Syrian rebels with weapons that included Manpads.

The only condition the Obama administration made at the time was that the Secretaries of State and Defence provide a report to Congress before doing so – not, most observers agree the most daunting of obstacles or restrictions.

Right now in the shadowy world of Syrian oppositional intrigues, where al-Qaeda-aligned militias collaborate with openly pro-Western groups and the CIA and US special forces, the covert supplies of weapons is a labyrinthine process to say the least. No outside player is squeaky clean.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 the risk of what intelligence agencies call “blowback” from terrorism carried out by weapons they supply has always been a gamble. Few weapons carry the risk of such devastating blowback more than Manpads.

But. putting aside for now those pillaged from captured Syrian arsenals, just where do those finding their way into jihadist hands come from?

According to Charles Lister of the Brooking Institution, the first were transferred to the Syrian opposition in at least three waves. Once in 2012, once 2013 and once in late 2015, most likely with involvement from Saudi Arabia and Qatar.

Since then things have move on considerably in the supply of arms to opposition forces, who regard these missiles as a game-changing weapon against Syrian and Russian air strikes.

As a result most have been engaged in demonstrating increasingly sophisticated and aggressive approaches towards acquiring these weapons. These might range from grey and black market sources, arms dealers, front companies, trans-shipment, intermediaries, end-use certificate falsification, and corrupt government officials. Few doubt that such networks are being used by the intelligence services of a number of countries supporting the opposition.

There would, for example, be nothing to stop the United States shipping them through a third party such as Saudi Arabia or Turkey.

There is nothing illegal in this, and what those countries do with them is considered their business, while end-use certificate constraints for many weapons have always been prone to manipulation.

According to the online magazine Middle East Eye, as far back as September 2016 a high-level source said that the US was clearing the way for allied nations to begin supplying Manpads, as Russian and Syrian forces increased their attacks on rebel-held areas on the northern Syrian city of Aleppo.

The source also told MEE that the US had confirmed it would allow Qatar and Saudi Arabia to begin shipments.

“The US confirmed the green light to begin sending them to rebels through supply routes still open through Jordan and Turkey,” the source told MEE around the time Obama signed the annual defence policy bill.

“Rebels are being told only to target Syrian helicopters, not Russian, but it’s not clear they will abide by this.”

In an article entitled “Who pulled trigger on Russian jet in war zone bristling with arms?” the Saudi-based Al Arabiya network pointed out that a number of Syrian opposition militias now have access to the weapons.

It said one type, US FIM-92 Stingers, were manufactured under licence in Turkey by Roketsan, a major Turkish weapons manufacturer and defence contractor. These, according to US-based analyst Theodore Karasik, had been delivered to “many Syrian opposition forces, like the Western-aligned Free Syrian Army (FSA), around Idlib”. The same province in which the Russian plane was shot down.

The type of portable missiles largely used in Syria continue to be of Soviet or Russian production origin, such as the “Igla” and “Strela”, which were found in the looted arsenals of the Syrian regime. Over many years – and long before the current Syrian war – Moscow was happy to supply Damascus, its ally in the region.

Others are likely to be Libyan in origin, among the many that went missing after the toppling of Libyan dictator Muammar Gaddafi in 2011. Such was the fear then that the CIA is said to have launched a concerted effort to try and track down and recover them, though not before some of the weapons found their way from Libya to Syria via Turkey.

Such is the existing double standards over Manpad use and supply that US officials have even talked of designing some version of the weapon that it would be willing to entrust to rebel groups, despite the risks of them falling into terrorist hands.This would involve components that make them inert after a period of time or the user having to insert a code before use.

One such 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 them using improvised batteries.

When asked by journalist Elias Groll of Foreign Policy magazine in 2016 about such research to modify them, Raytheon, which makes Stinger, the primary version in the American arsenal, the company declined to comment.

The US arms giant also makes laser guidance systems for Paveway IV missiles and has a major plant in Glenrothes in Fife.

Campaigners against the arms trade argue that surely rather than trying to modify the weapons to make them terrorist-proof, the real aim should be to stop them getting into their hands in the first place.

Right now that possibility is terrifyingly greater than ever before, as the proliferation of Manpads in Syria escalates and major powers manoeuvre and intrigue to boost their presence and influence freeing up the weapons for use in the process.

For decades terror groups have been trying to use ground-to-air missiles to take down civilian planes. With an altitude range of 10,000 to 15,000 feet they would make airliners especially vulnerable during take-off and landing.

In 2002 the attempted shooting-down of an Israeli passenger jet taking off from Mombasa, Kenya, was one of the last major warning of the weapons threat and capability. In the space of the intervening 16 years that threat has multiplied many times over.

For most of the major powers now embroiled in Syria the risk of blowback from terrorism carried out by weapons they supply has always been a gamble. Ever-increasing numbers are now unaccounted for.

“The problem is, once you set this genie out of the bottle it is much harder to control,”

warned Peter Bouckaert, Emergencies Director for Human Rights Watch, a few years ago as the threat from Manpads began to grow.

“There are very grave risks. It is the number-one weapon on the terrorist shopping list.”