Sunni Islamist extremists are the main suspects behind the bomb attack in Lebanon this morning and a deadly explosion in Damascus over the weekend, which many saw as revenge for Syria's long dalliance with jihadist elements.

While the explosion in Tripoli targeted Lebanese troops there for the second time in two months, the 200kg bomb that killed 17 people near a military installation and Shia shrine in Damascus was the worst such attack since the secular Syrian regime fought bloody battles with the Muslim Brotherhood in the 1980s.

Analysts believe that both attacks bear the hallmarks of terror groups linked to al-Qaeda intent on destabilising the region. Syria has offered shelter to such Mujahidin since the US invasion of Iraq in 2003, providing them a base.

But Syria, whose stability has long been enforced by its oppressive police and secret services, has been showing signs of strain in the past year, with a number of mysterious assassinations raising questions about internal power struggles and external threats, as it attempts to play all sides off against each other in a volatile region.

It has hosted dangerous Sunni Islamist extremists despite being a secular Sunni country headed by Alawites, a Shia sub-sect considered heretics by al-Qaeda and its hardline Sunni allies.

Syria denounced the Tripoli blast as a “terrorist and criminal act,” and expressed “its solidarity with brotherly Lebanon in the face of parties who are undermining the country's security and stability”. President al-Assad recently gave warning of violence from jihadist militants in northern Lebanon and called on the Lebanese army to rein them in. Since May, Sunni militants in northern Lebanon have clashed with the small Alawite community.

Syria recently deployed several thousand troops along Lebanon's northern border, sparking fears in Beirut that Damascus was contemplating a military incursion. Syria said that it was nothing more than an anti-smuggling drive.

Syria's state-run al-Thawra newspaper suggested that the perpetrators of the Damascus bomb attack had come from another country. “Syrian security is solid but the region is throbbing with terrorists,” it said. “We need to protect our frontiers to prevent infiltration by terrorists, explosions and acts of sabotage.”

Andrew Tabler, the editor of Syria Today magazine, said: “If Syria is cracking down on jihadis along the Iraq border and along the Lebanon border, then it would not be surprising if the jihadis strike back.”

In the past year, Syria has been shaken by a spate of assassinations and security breaches. In February, Imad Mughniyah, the military commander of Lebanon's militant Shia Hezbollah, was killed by a car bomb in Damascus. His death was a severe embarrassment to the regime and allegedly prompted a purge of the Syrian security apparatus.

In May, Syria and Israel announced they were holding indirect peace talks brokered by Turkey, but less than a week later, Damascus signed a fresh military co-operation agreement with Iran, Israel's arch enemy.

In August, a top Syrian general with links to Hezbollah was assassinated, while an aide, the Damascus-based Hamas chief Khaled Meshaal, was shot dead in a street in Homs. This month, a Hezbollah official in Damascus was reportedly killed in a car crash in Syria. Hezbollah said he had died while “on duty”.

There have been rumours of a possible split within the regime between those supporting peace talks with Israel and rapprochement with the West, and those who prefer to maintain the strong ties with Iran.

Iran and Hezbollah have much at stake in the future direction of Syria, which is the strategic linchpin connecting Tehran to its Hezbollah allies.

“Syria is at a crossroads,” said Elie Aoun, an anti-Syrian MP in Lebanon. “Syrian-Israeli talks are serious and achieving progress and we could soon hear that the negotiations have developed into the stage of direct talks ... Syria would have to choose between its pan-Arab belonging and the relationship with Iran.”

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