AL QASR, Lebanon -- Masked men in camouflage toting Kalashnikov rifles fan out through a dusty olive grove, part of a group of Hezbollah-backed fighters from Lebanon who are patrolling both sides of a porous border stretch with Syria.
The gunmen on the edge of the border village of al-Qasr say their mission is to protect Shiites on the Syrian side who claim their homes, villages and families have come under attack from Sunni rebels.
Hezbollah chief Sheik Hassan Nasrallah, leader of many of Lebanon's Shiites and a staunch ally of Syrian President Bashar Assad, has said his group is supporting the cadres of fighters who call themselves Popular Committees.
It is confirmation that the powerful Lebanese militant group is playing a growing role in the civil war just across the border.
Syria's regime is dominated by minority Alawites -- an offshoot of Shiite Islam -- while the rebels fighting to overthrow Assad are mostly from the Sunni majority. Assad's major allies, Hezbollah and Iran, are both Shiite.
The sectarian tensions in the civil war have spilled over to neighbouring Lebanon, which has a similar ethnic divide and a long, bitter history of civil war and domination by Syria. Deadly gunbattles have broken out in Lebanon in recent months between supporters of both sides of the Syrian war.
But more broadly, Hezbollah's deepening involvement shows how the Syrian civil war is exacerbating tensions between Shiites and Sunnis around the Middle East.
Syrian rebels accuse Hezbollah of fighting alongside Assad's troops and attacking rebels from inside Lebanese territory.
In recent months, fighting has raged in and around several towns and villages inhabited by a community of some 15,000 Lebanese Shiites who have lived for decades on the Syrian side of a frontier that is not clearly demarcated in places and not fully controlled by border authorities. They are mostly Lebanese citizens, though some have dual citizenship or are Syrian.
Before Syria's uprising erupted two years ago, tens of thousands of Lebanese lived in Syria.
The Lebanese Shiite enclave on the Syrian side of the border is near the central city of Homs and across from Hermel, a predominantly Shiite region of northeastern Lebanon.
One commander of the Popular Committees said Shiite villages have been repeatedly attacked and some residents have been kidnapped and killed by rebels. He said that prompted local Shiites to take up arms to defend themselves.
"We are in a state of defence. We don't take sides (between rebels and regime forces). We are here to defend our people in the villages," said the commander, Mahmoud, who gave only his first name out of fear for his own security.
"We don't attack any area. We only defend our villages."
The border region near Homs on the Syria side is strategic because it links Damascus with the coastal enclave that is the heartland of Syria's Alawites and is also home to the country's two main seaports, Latakia and Tartus.
One of the biggest battles in the area was on Thursday when the Syrian army captured Tal al-Nabi Mindo, a village near the Lebanese border, after a day of heavy fighting.
Mahmoud said there were casualties on both sides, adding that the hilltop village overlooks several towns and villages as well as a strategically important road that links Tartus to Homs and the capital of Damascus beyond.
Mahmoud said some rebel commanders were killed in the fighting on Thursday and rebels threatened to bombard Lebanese territory in retaliation.
On Sunday, two rockets fired from Syria exploded in al-Qasr, killing one person, a Lebanese security official said on condition of anonymity because he was not authorized to brief reporters. Two more rockets landed in a nearby village of Hawsh, killing a 13-year-old boy and damaging two homes, the official said. It was unclear who fired the rockets from Syria, the official said.
The Popular Committees were set up last year with the backing of Hezbollah. But even though Hezbollah confirms backing the fighters, it denies it is taking part in the wider civil war.
Syrian rebels offer a different narrative, accusing Hezbollah of propping up the Assad regime.
"Hezbollah is involved in the war that the Syrian regime is launching against the Syrian people," said Loay al-Mikdad, a spokesman for the rebel Free Syrian Army (FSA).
In the past two months, he said Hezbollah has expanded its operations in Syria, mostly in central Homs province near the Lebanese border, as well as in Damascus.
He claimed that Assad is relying on Hezbollah because his grip on the capital is weakening and he fears more military defections.
"(Assad) had to depend on militias such as Hezbollah to defend his regime," al-Mikdad said. He said Hezbollah is defending the holy Shiite shrine of Sayida Zeinab, named for the granddaughter of Islam's Prophet Muhammad's, south of Damascus. Hezbollah militants are also fighting elsewhere in the capital, he claimed.
The Popular Committees are just one indication of Hezbollah's role in the Syrian civil war.
Over the past several weeks, the group has held several funerals in Lebanon for gunmen who it said were killed while "performing their jihadi duties." It did not say where or how they were killed, but it is widely known they died fighting in Syria.
One of the biggest blows for Hezbollah in Syria came in October when a commander, Ali Hussein Nassif, also known as Abu Abbas, and several other fighters were killed. Syrian rebels said his car was hit by a bomb near the Syrian town of Qusair, close to the Lebanon border.
"The impact of Hezbollah on the conflict should not be underestimated," said Torbjorn Soltvedt, senior analyst at the British risk analysis firm Maplecroft.
"Crucially, the group is much more adept at fighting the type of irregular conflict that is taking place in Syria than the Syrian armed forces, which have been trained and equipped primarily to fight conventional warfare."
Hezbollah fought guerrilla warfare against Israel for nearly two decades until 2000 when Israel withdrew from an enclave it occupied in south Lebanon.
The militant group's staunch support for the Assad regime is a gamble. Hezbollah's image in the Arab world as a resistance force against Israel is already eroding.
Hezbollah backed the wave of Arab Spring uprisings against autocratic rulers in Egypt, Yemen, Bahrain and Tunisia, but publicly sided with Iran and Syria in their crackdowns on protesters.
Assad's fall would be catastrophic for the group. Any post-Assad regime led by Syria's Sunni majority would almost certainly be far less friendly -- or even outright hostile -- to the Shiite group.
Iran remains Hezbollah's most important patron, but Syria is a crucial supply route. Without it, Hezbollah will struggle to secure the weapons it needs to fight Israel.
Hezbollah maintains its own separate arsenal that is the most powerful military force in Lebanon, stronger than the national army. In addition, the country of 4 million has dozens of smaller militias allied with political factions.
Assad's fall would probably ratchet up pressure on Hezbollah at home, where the group's anti-Syrian rivals have long demanded the Shiite group disarm its militia -- tens of thousands of fighters with long-range missiles.
Hezbollah insists the weapons are necessary to defend Lebanon against Israeli attack and refuses to disarm.
Soltvedt, the analyst, said support in the form of fighters and training is unlikely to be enough to prevent Assad's eventual fall. But he said it has helped the regime hold out.
"Hezbollah's involvement in the conflict has undoubtedly strengthened the regime's ability to combat the rebels and prolonged the conflict," he said.
As a result of the tensions, hundreds of Lebanese Shiite families in Syria have fled back to their homeland.
A few months after the revolt began, Safiya Assaf, her husband and their 11 children fled Qusair near the border to safety in al-Qasr just across the frontier. They left behind three homes and three shops.
"They (rebels) sent us a threat with a person from the area ordering us to leave ... because we are Shiites," said Assaf, sitting on a mat and surrounded by some of her children, grandchildren and a daughter in law in an apartment they are renting in al-Qasr.
Bilal al-Sadr, another villager, lived in Syria for 14 years before deciding to flee with his wife, four sons and a daughter. He left after three of his friends -- a Sunni, a Shiite and a Christian -- were kidnapped and killed.
"My home and shop were burnt and family threatened," said al-Sadr, a Shiite Jordanian whose mother and wife are Lebanese from al-Qasr. "When we felt that our safety was in danger, we decided to leave."
Back on the border, a Popular Committee member said Shiite residents in Syrian border villages have no choice but defend themselves.
"Do you expect us to wait for al-Qaida to come and slaughter us?" asked the masked fighter.
