Free Syrian Army fighters carry their weapons as they sit on the ground. Recent clashes between Syrian rebel fighters and Lebanese Hezbollah militants inside Syria have ramped up tensions on both sides of the border. (Zain Karam/Reuters)

Fierce clashes between Lebanese Hezbollah militants and rebel fighters inside Syria have ramped up tensions on both sides of the border and could spill over into Lebanon, potentially starting a new round of sectarian bloodletting.

Fighting near the Syrian city of Qusayr two weeks ago, the heaviest since the conflict began nearly two years ago, left at least two Hezbollah fighters and more than a dozen rebels dead, according to Lebanese officials and rebel fighters.

 And for the first time, Syrian rebels have threatened to take the fight to Hezbollah inside Lebanese territory, a potentially dangerous expansion of the conflict.

 Hezbollah is the most powerful political and military force in Lebanon. It is also a group supported primarily by Shiite Muslims, many of whom back the government of Syrian President Bashar al-Assad. The opposition in Syria is made up of mostly Sunni Muslims and has broad support among Sunnis in Lebanon.

 Sunni militant groups from Lebanon have also been sending fighters into Syria and giving weapons and logistical support to the opposition, according to Lebanese security officials.  Now, tensions are peaking as Shiites and Sunnis from Lebanon are fighting one another inside Syria, increasing the potential for conflict back home.

 The fighting on the border could easily spread to Tripoli, Sidon or even Beirut, cities where heavy clashes in the past two years between Shiites and Sunnis linked to the Syrian conflict have left dozens dead.

 The conflict also has regional implications. Hezbollah has linked with Iran to form an axis of Shiite support for Assad, whose Alawite sect is an offshoot of Shiism, while Turkey, Qatar and Saudi Arabia have formed a grouping of predominantly Sunni powers that support the opposition.

In a report to the U.N. Security Council last week, U.N. Secretary General Ban Ki-moon expressed “grave concern” about the “further deaths of Hezbollah members fighting inside Syria” as well as reports of Sunni Lebanese fighters being killed in Syria.

 “The dangers for Lebanon of such involvement and indeed of continued cross-border arms smuggling are obvious,” he said. “I call upon all Lebanese political leaders to act to ensure that Lebanon remains neutral in respect of external conflicts.”

In a speech Wednesday, Hezbollah leader Hassan Nasrallah acknowledged that there are Hezbollah militants fighting in the villages near the border and said the residents there have a right to defend themselves. He also issued a stern warning, saying, “No one should make any miscalculations with us.”

Observers say that one misstep from Shiite or Sunni militants on Lebanese soil could cause the situation to deteriorate quickly. 

“They are involved in the war on the other side, but they are trying not to bring it to Lebanon. The problem is, this thing can collapse at any moment,” said Timur Goksel, a former senior adviser to the U.N. monitoring team in Lebanon who is a political science lecturer at the American University of Beirut. “We may have a very serious outbreak of violence in Lebanon. So that’s what is scary.”

‘It is Lebanese land there’

The area where the most recent clashes took place last week includes roughly 22 villages and is home to about 30,000 people in western Syria who are predominantly Shiite and have deep historical and family ties to Lebanon. As the Syrian opposition moved into the area in recent months,some families fled to Lebanon. Others hunkered down for a fight.

On the Lebanese side of the border, there is little doubt who is in charge: The yellow-and-green flag of Hezbollah flies atop many buildings, and pictures of the group’s martyrs hang from banners.

The influence of Iran is also hard to miss. A broad, smooth highway, a rarity in the area, leads into Hermel, one of the biggest towns near the border. The highway was reconstructed with Iranian funds after Hezbollah’s 2006 war with Israel. Signs on lampposts thank the Islamic republic for its support.

As clashes in the contested villages intensified two weeks ago, Lebanese fighters, many of them affiliated with Hezbollah, crossed the border to join the battle, local officials said.

“It is Lebanese land there. They are all supporting Hezbollah,” said Issam Bleibel, the deputy mayor of Hermel and a Hezbollah member. “It’s natural for people from here to go and defend them there.”

Opposition fighters in Syria accuse Hezbollah militants of displacing Sunnis from the border area.The area has great strategic value for the Syrian opposition because Lebanese Sunnis help smuggle men and materiel across the border, according to a Lebanese former senior security official who, like others interviewed, spoke on the condition of anonymity for security reasons.

Not only are Hezbollah fighters coming across the border, rebel fighters say, but the group is also shelling their positions from the Hermel area.

Bleibel, the deputy mayor, denied the assertion of shelling from Lebanon but said the Syrian military regularly shells rebel positions.

An existential fight

 Rebel fighters say many of the Hezbollah attacks are timed to support Syrian military operations. “The latest attack was in coordination with the regime army,” said a spokesman for the Free Syrian Army in Qusayr who uses the nom de guerre Jaad Yamani. “Whenever there is pressure on the regime, Hezbollah moves in to help.”

Last week, FSA commanders issued an ultimatum to Hezbollah to stop cross-border shelling and later said they had launched an attack on Hezbollah artillery positions in the village of Housh Seyed Ali, a farmland area in Lebanon.

 Residents of Housh Seyed Ali, from where the contested villages a few miles away in Syria are easily seen, say there was no attack from the Syrian rebels. But one resident, who spoke on the condition of anonymity, said the intense shooting and explosions from the recent fighting had terrified his children.

 Still, two rockets fired from Syria recently hit the nearby village of Qasr.

 For both sides, it seems to be an existential fight for their respective sects. Syrian opposition fighters say Hezbollah is trying to shift Sunnis away from the border area to create a sectarian enclave linking the Shiite villages to the traditional Alawite heartland in western Syria, a charge that Nasrallah denied in his speech Wednesday.

 And Syrian Shiites near the Lebanese border say they feel an imminent threat from radical Sunnis fighting in the Syrian opposition. Hassan Sakr, a 43-year-old lawyer who is Shiite, fled to Hermel from Qusayr more than a year ago because of the threat from Sunni fighters.

“Unless there is an international agreement, there will be no solution,” Sakr said. “There are lots of weapons and lots of fighters, and they will keep fighting.”

Suzan Haidamous and Ahmed Ramadan contributed to this report.