Portraits of Hezbollah fighters slain in the weekend battle for the Syrian city of Qusair dotted the roads of villages in northern Lebanon on Monday, evidence of the militant group’s increasing entanglement in the Syrian civil war.

The bodies transported back over the border point to heavy losses for the Lebanese Shiite movement since its fighters backed Syrian President Bashar al-Assad’s forces in an assault on the strategic city, which lies about six miles from the border. Opposition groups say that at least 28 Hezbollah members were among 90 people killed since Sunday.

The involvement of Hezbollah threatens to embroil Lebanon in the Syrian civil war, but it also could help tip control of Qusair back to the regime. The fall of the city could prove pivotal in the two-year-old conflict, providing a staging point for an assault on the central rebel bastion of Homs and assisting the regime in securing a crucial pathway between the capital, Damascus, and the Alawite heartland along the Mediterranean coast, which includes the ports of Latakia and Tartus.

“To lose Qusair would be a disaster; we will lose the whole city of Homs,” said Fadi al-Issa, a fighter with the opposition Farouq Brigade who is recovering in the border village of Aarsal after his leg was shattered in shelling just outside Qusair three months ago. But he fears heavy airstrikes teamed with the ­Hezbollah-backed ground assault have put the opposition on the defensive. “Hezbollah are well trained in street fighting, and they are using their elite,” he said.

There are concerns that the entrenchment of the Iranian-backed militant group could inflame a regional struggle between Sunni and Shiite Muslims. Hezbollah militants, trained to battle the “Zionist enemy,” are known for their prowess in guerrilla warfare, and the group has bolstered its arsenal since a war with Israel in 2006.

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

But its new struggle against fellow Muslims has proved controversial at home. A Hezbollah spokesman declined to comment on the number of fighters killed since Sunday in the Syria fighting, but the group’s deepening role in the conflict is difficult to hide.

A Hezbollah checkpoint blocked access to the center of the largely Shiite village of Labweh in the Bekaa Valley as the body of a local fighter was returned home Monday. Banners hailed the militant’s martyrdom in “jihadist duties” as young girls dressed in black thronged the streets and men on scooters draped in yellow Hezbollah flags buzzed through the traffic.

In the nearby town of Nabi Sheet, the Associated Press reported that about 2,000 people attended the funeral of 18-year-old Hezbollah fighter Hassan Shukur, firing weapons in the air as his casket was carried through the streets. He was among those reported killed in the fighting Sunday.

Losses on the rebel side also have been hefty. Qusair has been under siege for weeks, with pro-regime forces seizing control of surrounding villages one by one. One of his brothers was killed four days ago, Issa said, and another, also a fighter with the Farouq Brigade, called in Monday with an update from the battlefield.

“We are on defense,” Mohammed al-Issa said by telephone from the bombarded city. He estimated that as many as 70 rebels have died since Sunday, but he said counts were difficult with bodies still trapped under rubble. The rebel Free Syrian Army repelled a push by pro-
regime forces heading toward the municipality building Sunday, he said. They attempted another offensive Monday but only made it to within 700 yards of the center before retreating, he said.

However, the state-run Syrian Arab News Agency said army units secured control of the eastern part of the city “after killing big numbers of terrorists,” the term the government uses to describe the rebels.

Fleeing residents said that about 20,000 people remain trapped in the city, which had a population of more than 35,000 before the conflict erupted. With the town surrounded by pro-
regime forces, escape has been risky for weeks, but the army’s closing of a route on the eastern side of the city several days ago has made fleeing nearly impossible.

One 30-year-old resident said he had walked for about 16 miles before arriving in Aarsal two days ago. Khanj al-Khanj, 56, said he’d had to crawl for 300 yards in the dark to avoid being shot at by pro-regime forces patrolling the border.

Khanj owns five farms in the Qusair countryside, and he said several had been taken by Assad’s forces in recent weeks for use as bases. “There’s no power, there’s no water, no communications,” he said. “The killing is everywhere. We’ve been burying three to four martyrs to a grave because there is no space.”

President Obama called his Lebanese counterpart, Michel Suleiman, on Monday to discuss Lebanon’s stability, as fears grow over how long Syria’s fragile neighbor can insulate itself from the war next door.

Assad’s opponents and supporters live cheek by jowl in Lebanon, where the population is sharply divided over the Syrian conflict. A Lebanese soldier was killed in fighting between pro- and anti-Assad factions in Tripoli, Lebanon’s second-largest city, on Sunday.

The influx of refugees is further threatening the country’s delicate security situation. Just a few miles down the road from Labweh, in the village of Jdeideh, Nawal Baker greeted guests with the bitter black coffee traditionally served by families in mourning.

“We are living among the enemy,” the 39-year-old Syrian said. She described how her son, Fouad, was killed 10 days ago on the outskirts of Qusair. She is unwavering in whom she holds to blame. “Hezbollah killed my son,” she said.