— Sunni militants have been flocking from Lebanon to Syria in greater numbers in recent months to join forces with Islamic 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 is the al Nusra Front , a militant group thought to have links to al-Qaeda that the U.S. government has labeled a foreign terrorist organization, according to senior Lebanese security officials.

The al Nusra militants have established links with extremist cells mostly based out of Tripoli, Lebanon’s second-largest city, which has long been 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 who asked not to be identified because he is not authorized to speak on the record.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

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 alone, 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 has emerged from an episode in late November, when a group of 22 volunteers sympathetic to the Syrian opposition crossed the border from north Lebanon into Syria. The majority of the group were young Lebanese men though there were also some Palestinians and Syrians living in Lebanon among them, the officials say.

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 asked not to be identified because they are not authorized to speak on the record.

Nineteen of the men were killed, and a video was posted online shortly after the attack showing gunmen kicking and cursing the corpses of the fighters.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

The deaths sparked several days of clashes between Sunni and Alawite fighters in Tripoli in early December, leaving at least 12 dead and dozens more wounded.

The episode followed months in which extremist Sunni leaders in Tripoli had been calling on followers to increase their support for their counterparts across the border, according to the Lebanese officials.

Small groups were organized in north Lebanon to facilitate the transport of weapons, ammunition and logistical equipment, as well as fighters, across the border into Syria with the help of smugglers. These groups initially communicated with their Syrian counterparts with cellphones but eventually began using more sophisticated and secure communication methods, such as Thuraya satellite phones.

Militant leaders well known

Two of the leading figures who are helping expand the ties between Lebanese and Syrian extremists groups have been known to Lebanese security officials for years.

On the Lebanese side of the border, the trip for the volunteers killed in Tel Kalakh was partially funded and organized by Hussam Sabbagh, a well-known militant who is thought to have fought in Afghanistan, according to a senior Lebanese security official who asked not to be identified because he is not authorized to speak on the record. Sabbagh, contacted through intermediaries, refused to give an interview.

The main point of contact on the Syrian side was Khaled 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 toting machine guns.

Mahmoud, using the nom de guerre Abu Suleiman al Muhajer, described several religious injunctions to urge Muslims to wage jihad 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 as the emir or religious leader of the group in the video, said that they would be operating 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 of the leaders of Fatah al Islam were either killed or imprisoned in Lebanon’s notorious Roumieh prison after the clashes. Mahmoud served seven years in the Roumieh prison for his militant activities and was released only last summer. He crossed the border into Syria with the help of smugglers shortly after he was released from jail, according to a senior Lebanese security official.

‘Burden for the opposition’

Sunni religious leaders in Tripoli, for their part, say the ties between militants in Syria and Lebanon are exaggerated and 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 Sheikh 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 with opposition flags waving from rooftops and graffiti calling for the ouster of Assad.

The main flash point for the factions supporting and opposing 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 neighborhood, from Jebel Mohsen, a predominantly Alawite neighborhood.

The Lebanese army has set up armored personnel carriers every few blocks on the street and have been able to keep the peace in recent weeks. But unrest has flared in other parts of the city, as well.

“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.”

Ahmed Ramadan contributed to this report.