Lebanese soldiers stormed a complex holding gunmen loyal to a radical Islamist cleric in the city of Sidon on Monday and arrested dozens of his supporters, security sources said, in a second day of clashes fuelled by neighbouring Syria's civil war.

The fighting is the deadliest outbreak in Lebanon since Syria's two-year conflict began. The army said 12 soldiers were killed in the southern Mediterranean port city, while security sources gave a higher army toll of 18 dead and 128 wounded.

A medic said 22 bodies had been pulled from the mosque complex but he expected the final death count to be higher. He said 94 wounded had been treated by the Red Cross.

The violence has strained fragile sectarian relations across Lebanon and residents fear that Syria-related clashes could drag their country back into civil war. Lebanon is still struggling to heal the wounds of 15 years of war between 1975 and 1990.

Fighting spread to a second city on Monday, with Sunni fighters in the northern city of Tripoli opening fire on the military and blocking roads with cement blocks and burning tires. By nightfall, clashes there had injured two soldiers and 
three gunmen.

Hardline Sunni cleric Sheikh Ahmed al-Assir, who has accused the army of backing the interests of the Shi'ite Muslim group Hezbollah, was still at large after the battle in Sidon. 

The army is trying to kill or capture him, accusing him of killing soldiers "in cold blood" on Sunday.

Security forces were taking over houses around the mosque as they tried to control the area in Sidon. Clouds of smoke rose from the mosque and Assir's office across the road was completely destroyed. At least four tanks and several army vehicles at the scene had been torched.

Sniper fire continued to hit nearby streets, so it was unclear how many buildings Assir's gunmen still controlled.

Sidon had been on edge since violence erupted last week between Sunni and Shi'ite Muslim fighters, at odds over the Syrian conflict which pits mainly Sunni rebels against President Bashar Assad, who is a member of the minority Alawite sect, an offshoot of Shi'ite Islam.

Tensions had been rising since the Lebanese Shi'ite militant group Hezbollah sent fighters into Syria to help Assad's forces recapture a strategic town.

The army said clashes broke out on Sunday after security forces detained one of Assir's followers. His supporters retaliated by opening fire on an army checkpoint.

Army commanders pledged to crush Assir's forces.

"We affirm to all Lebanese that the army is determined to eradicate strife, and will not halt its military operations until security is completely restored to the city," the army said in a statement on Monday.

At least 62 Assir supporters were arrested as soldiers combed the area they had seized, a security source said. One of the men captured had disguised himself in niqab, the head-to-foot cloak worn by religious Muslim women.

The commissioner of Lebanon's military court, Judge Sakr Sakr, said that Assir had been summoned "to be put on trial, along with 123 of his followers, including his brother and Fadil Shaker," a prominent Lebanese singer who abandoned his career to join Assir's ultraconservative group.

"Save your people"

"Come and save your people who are being massacred," said an appeal on Assir's Twitter account on Monday. The bespectacled sheikh with a long grey beard was little known before the Syria conflict but quickly rose to fame for his
protests and marches in support of the rebels and Islamist groups in Syria.

Media took note when he made a brief visit to the besieged Syrian border town of Qusair, where Hezbollah led Assad's forces to victory this month.

The cleric and his followers have grown increasingly hostile in their condemnations of Hezbollah and its Shi'ite supporters, and skirmishes in Sidon were becoming more common.

Assir, who accuses the army of giving cover to Hezbollah gunmen, called in a YouTube video for people across the country to join him and "honourable" soldiers to defect. He was pictured carrying a gun, in a black military-style vest.
A statement by the military command on Sunday said the latest violence "has gone beyond all expectations. The army was attacked in cold blood in an attempt to light the fuse in Sidon, just as was done in 1975", it said.

Assir has a small group of staunch supporters, believed to number in the hundreds. Many other Lebanese Sunnis see him as a militant and trouble maker.

Lebanon's Grand Mufti Mohammad Rashid Qabbani condemned the fighting, saying that there was never a justified reason to attack the armed forces.
However, local media reported that some hardline Sunni mosques in Tripoli as well as the capital Beirut called for jihad, or holy war, in support of Assir. Jihadi feeds on Twitter were full of calls for Sunnis to fight in support of him.