THE possibility of Syria's civil war spreading across the Middle East became increasingly likely when it was revealed that one of the victims of Friday's car bomb in Beirut was General Wissam al-Hassan, the head of Lebanon's internal security forces.

A Sunni Muslim, he was also an opponent of president Bashar al-Assad's Syrian regime. Earlier in his career he had led the investigation into the assassination of former Lebanese prime minister Rafik al-Hariri, who was killed in a car bomb attack in 2005.

Hassan's findings back then implicated Syria and Hezbollah, and yesterday Lebanese politicians were quick to lay the blame for the general's assassination on neighbouring Syria. In the wake of the incident, Sunni Muslims took to the streets of Lebanon to protest, burning tyres in the capital, Beirut, and in the eastern Bekaa Valley region. Last night Walid Jumblatt, leader of the Druze sect and a critic of Damascus accused "Assad and his regime" of killing Wissam al-Hassan.

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Coming on top of last week's revelations that the casualties in the Syrian civil war might be higher than expected, this is the clearest sign yet that the fighting might be dragging neighbours into the conflict. Once again in this crisis – now in its 20th month – human rights groups have expressed fears about the high casualty rate, and especially about the fate of those non-participants who have disappeared during the fighting.

Checking the fate of a missing person is well-nigh impossible in any civil war. Not only do the violent conditions prevent a reliable investigation taking place at the time, but when family is turned against family and communications break down there is no incentive to tell the truth.

Last week the harsh reality of life in war-torn Syria became apparent when human rights groups made the astonishing claim that at least 28,000 people have disappeared after being abducted by government soldiers or militia fighters who form the opposition to the Assad regime.

Of that number, 18,000 people have been posted as missing by their families since the beginning of the rebellion and human rights groups say that they know of at least another 10,000 cases. At the end of last week an internet-based human rights organisation known as Avaaz posted a message saying that "nobody is safe" from a deliberate government campaign of terror.

To put these figures into context, they have to be compared to the casualties of the violence. According to statistics provided by the United Nations, more than 18,000 people have been killed in the fighting since last April with 170,000 fleeing abroad and 2.5 million in need of aid within the country. Terrible though these figures are, opposition groups estimate that the death toll could be much higher, at more than 30,000.

Recent experience of the civil war in Iraq is also instructive. This broke out in 2003 in the aftermath of the United States-led operation to overthrow president Saddam Hussein – in the bloody confusion, thousands of innocent people simply disappeared. It was not until the fighting died down and a sense of normality returned to the country towards the end of the decade that figures could be checked, but despite intricate research the exact final figure is unknown. According to Iraq's Missing campaign, it could range from hundreds of thousands to more than one million people.

What happened in Iraq at the beginning of this century is now being played out again in Syria as rival groups take advantage of the chaos to settle old scores. One ploy is to use families as hostages in neighbourhoods where rivalries were kept under check by the authorities and by the sense of "live and let live" which characterises so many communities in the Middle-East with their mix of religions and races. But with Syria in meltdown as a result of the internecine fighting, both sets of armed fighters have used the unrest to their own advantage.

Alice Jay, the campaign director of Avaaz, has revealed the existence of a Wild West mentality in which Syrians were simply being lifted off the streets by armed thugs – government or paramilitary – in the major towns and cities and have never been seen again. The most likely outcome is death or imprisonment in hidden torture cells where their fate might never be known.

'Whether it is women buying groceries or farmers going for fuel, nobody is safe," she says, arguing that it is a deliberate policy of terror aimed at cowing the population of Syria. "The panic of not knowing whether your husband or child is alive breeds such fear that it silences dissent. The fate of each and every one of these people must be investigated and the perpetrators punished"

All the evidence points to government soldiers instigating the policy in the early days of the fighting as they attempted to gain control in key Sunni-dominated cities such as Dera'a, Homs and Hama, where the populations were subjected to physical violence and intimidation as the government attempted to gain control.

Kidnapping and arrest became weapons of war as people disappeared leaving their families without any recourse to intervention by the police. Those undertaking most of the kidnaps are known as "shabiha" or "ghosts". They are mainly members of the internal security forces who act with impunity – with the direct encouragement of the government army.

The aim is not just to intimidate but also to remove people who might have witnessed atrocities or who were known opposition activists. Religion also comes into it: most of the disappeared are Sunni, while their abductors are mainly Shia or members of the ruling Alawite minority.

Another human rights campaigner, lawyer Muhammad Khalil, from the north-east city of Hasaka, believes that this is a deliberate government-inspired policy aimed at achieving the government's goal of crushing all opposition.

He said last week: "While there is no precise figure, thousands of people have disappeared since March last year. The regime is doing this for two reasons: to directly get rid of the rebels and activists, and to intimidate the society so that it won't oppose the regime."

Opposition groups have also used the same tactics, kidnapping innocent people who might be useful or who are simply on the wrong side of the religious divide.

And no sooner had the conflict begun than it was radicalised with fundamentalists and jihadi fighters arriving in the country to support the rebels. Salafist groups such as Jabhat al-Nusra ("The Support Front") are closely linked with al-Qaeda-type groupings which see the conflict as a life-or-death struggle between the majority Sunnis and the Alawites supporting the Assad regime.

Removing the opposition is as good a way as any of cleansing the population of "heretics", many of whom have simply disappeared into thin air and as far as their families are concerned are as good as dead.

It has not only been in the Middle East that tactics such as these have been favoured by rival groups in periods of tension. The most notorious example in relatively recent times was the fate of "los desaparecidos" – "the disappeared" – in Chile after the military junta seized power in September 1971.

Under the subsequent rule of General Augusto Pinochet, thousands of Chileans were persecuted and many of them were taken into custody, never to be seen again. According to Thor Halvorssen, president of the Human Rights Foundation, Pinochet "shut down parliament, suffocated political life, banned trade unions, and made Chile his sultanate. His government disappeared 3000 opponents, arrested 30,000, torturing thousands of them."

The practice of kidnapping people during periods of civil conflict was also pursued in conflicts in Argentina and Colombia, and closer to home it was also a feature of the fighting in Northern Ireland during the Troubles of 1969-2007. Some 16 people were reported as having been abducted, killed and buried in unmarked graves by republican paramilitaries.

In 1999, the IRA admitted to killing nine of the 16, and produced information on the location of the bodies, but only ten have ever been recovered.

These examples show that uncovering the truth can only be achieved when the country concerned is stable once again – and yesterday Syria was still embroiled in fighting. With winter fast approaching there is a real danger that the casualty figures will soar as people try to escape from the fighting, taking their families and possessions over the border into neighbouring Lebanon and Turkey.

Fearing the worst outcome, Turkey's foreign minister Ahmet Davutoglu said that the only solution to the crisis was an immediate truce during the four-day Eid al-Adha holiday which begins on Friday. But given the events of the last few days, that possibility remains as remote as ever it has been.