Beirut: Sunni militants have been flocking from Lebanon to Syria in greater numbers in recent months to join forces with Islamist extremists battling the Syrian government, according to senior Lebanese security officials.

The escalating role that the Lebanese fighters are playing in the conflict is a direct result of expanding ties between Sunni religious extremists on both sides of the border and has raised concerns in Lebanon about a renewal of sectarian tensions.

At the forefront of the growing Sunni alliance, according to senior Lebanese security officials, is the Al Nusra Front, a militant group that is thought to have links to Al Qaida in Iraq and that the US government has labelled a foreign terrorist organisation.

The Al Nusra militants have established links with extremist cells based mostly in Tripoli, Lebanon’s second-largest city and long a hotbed of Sunni militancy.

“A strong relation exists between the Al Nusra Front command in Syria and Sunni extremists in Tripoli,” said a senior Lebanese security official. He spoke on the condition of anonymity because he was not authorised to talk on the record.

Many Lebanese Sunnis strongly support the opposition in Syria, while Lebanese Shiites mostly back the Alawite-led government of President Bashar Al Assad. Shiite militant groups in Lebanon, including Hezbollah — the most powerful military and political group in the country — have also sent fighters to Syria in recent months.

“Sunnis in Lebanon, whether they are extremists or not, whether they are religious or not, side very strongly with the Syrian uprising,” said Hilal Khashan, a professor of political science at the American University of Beirut. The divisions over Syria within Lebanon have played a role in widening sectarian clashes between Sunni and Shiite fighters in Tripoli as well as in the capital, Beirut.

In the past year, at least 70 people have died in such fighting. Lebanese security officials say the clearest example of the increasing links between Sunni militants in Lebanon and their counterparts in Syria occurred in late November, when a group of 22 volunteers sympathetic to the Syrian opposition crossed the border from north Lebanon into Syria. T

he majority were young Lebanese men, though there were also some Palestinians and Syrians living in Lebanon among them, the officials said. Only a few miles across the border, the group was ambushed by Syrian security forces near the town of Tel Kalakh and came under a hail of gunfire, according to the senior Lebanese security officials, who spoke on the condition of anonymity because they were not authorized to speak on the record.

Nineteen of the men were killed, and a video posted online shortly after the attack showed gunmen kicking and cursing the volunteer fighters’ corpses. The deaths sparked several days of clashes in early December between Sunni and Alawite fighters in Tripoli that left at least 12 dead and dozens more wounded. The episode came after months of calls by extremist Sunni leaders in Tripoli for greater support for Sunni fighters battling the Syrian government, according to the Lebanese officials.

Small groups were organised in north Lebanon to facilitate the transport of weapons, ammunition and logistical equipment, as well as fighters, into Syria with the help of smugglers. Those groups initially used mobile phones to communicate with their Syrian counterparts but switched to more sophisticated and secure methods, such as Thuraya satellite phones. Two leading figures who are helping expand ties between Lebanese and Syrian extremist groups have been known to Lebanese security officials for years.

The trip for the volunteers killed in Tel Kalakh was partially funded and organised in Lebanon by Hussam Sabbagh, a militant who is thought to have fought in Afghanistan, according to a senior Lebanese security official who spoke on the condition of anonymity because he was not authorized to talk on the record.

Sabbagh, contacted through intermediaries, refused to give an interview. The main point of contact on the Syrian side was Khalid Mahmoud, another well-known Lebanese militant, according to a senior Lebanese security official. In late December, Mahmoud appeared in a video posted online wearing a black turban and flanked by two masked men holding machine guns. Using the nom de guerre Abu Sulaiman Al Muhajer, Mahmoud in the video described several religious orders urging Muslims to wage holy war in Syria.

He also announced the formation of Jund Al Sham, the first Sunni armed opposition group in the Syrian conflict led by a Lebanese militant. Mahmoud, identified in the video as the emir or religious leader of the group, said it would operate in Homs province, which borders Lebanon. The ties between Sabbagh and Mahmoud go back many years. Both men had links with Fatah Al Islam, a radical Sunni group that fought a bloody battle against the Lebanese army in north Lebanon in 2007 that left at least 100 soldiers and militants dead.

Many Fatah Al Islam leaders were either killed or sent to Lebanon’s notorious Roumieh prison. Mahmoud was imprisoned in Roumieh for seven years for his militant activities. He was released last summer. Shortly afterward, he crossed the border into Syria with the help of smugglers, according to a senior Lebanese security official.

Sunni religious leaders in Tripoli, meanwhile, say that ties between militants in Syria and Lebanon are exaggerated and that the Syrian opposition does not need the help of Lebanese fighters. “We tell them not to go to Syria. They don’t need them there,” said Sheik Salem Rifai, a senior Sunni cleric in Tripoli.

“They don’t know the geography of the place, and they would need food and shelter, which would be a burden for the opposition in Syria.” Still, the residents of Tripoli publicly show their support for the Syrian rebels by flying opposition flags from rooftops and creating graffiti calling for the ouster of Al Assad.

The main flash point for the factions supporting and opposing Al Assad in the city has been Syria street, a congested thoroughfare with bullet-riddled shops and apartment blocks that separates Bab Tabbaneh, a predominantly Sunni neighbourhood, from Jabal Mohsen, a predominantly Alawite neighborhood.

The Lebanese army has set up armoured personnel carriers every few blocks and has been able to keep the peace in recent weeks. But unrest has flared in other parts of the city.

“The extremists proved to be in control, and the majority of the people support them,” the senior Lebanese security official said of the situation in Tripoli. “And the government cannot stop them.”