Six Lebanese soldiers were killed on Sunday in clashes with supporters of a radical Sunni Muslim sheikh opposed to the powerful Shiite movement Hezbollah, an army statement said.
"An armed group loyal to Sheikh Ahmad al-Assir attacked, for no reason, a Lebanese army checkpoint in the village of Abra" on the outskirts of the southern Lebanese city of Sidon, it said.
It said six soldiers were killed, two of them officers, updating an earlier toll of three dead.
The fighting erupted when Assir supporters surrounded an army checkpoint in Abra, where a vehicle transporting other supporters of the Sunni cleric had been stopped, a security source told AFP.
"After the armed men attacked with gunfire," the army fired back, the source added.
The army vowed it "will not tolerate" the latest developments, and that it "will continue to fulfil its mandate to suppress strife".
The military will "strike back with an iron fist anyone who... spills the blood of the army", the statement said.
Amjad Assir, the sheikh's brother, was defiant when reached by telephone.
"The army is with Hezbollah -- we're being bombarded from all sides," he told AFP as explosions were heard in the background.
"Sheikh Assir will stay on the battlefield along with those who support him. We will resist to the last drop of blood."
Sunday's fighting was the latest in Lebanon in a series of Syria-related confrontations.
Though Lebanon has officially adopted a position of neutrality on the conflict in its larger neighbour, it is deeply divided into pro- and anti-Damascus camps.
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Abra is home to a mosque where Assir leads the main weekly prayers on Fridays. The sheikh believes Hezbollah uses apartments in Abra to keep him under surveillance.
He also says the Lebanese army has provided cover to Hezbollah, whose members are fighting alongside Syrian President Bashar al-Assad's troops against rebels.
Assir was unknown until around two years ago, when he gained prominence for his opposition to Hezbollah and its ally Assad.
The military urged political leaders in Sidon to choose sides and to stand either alongside the army, or with "those who promote strife and the killing of troops".
It also said the army was targeted "in cold blood", despite trying "for months to distance Lebanon from the situation in Syria".
Shortly after the clashes broke out, an AFP correspondent in Sidon saw civilians fleeing the fighting, both by car and on foot.
Smoke could be seen rising above Abra, especially from the area where Assir is based, as more troops were deployed.
Some Assir supporters fled, and the military pursued them through the streets of Sidon, south Lebanon's main city.
The national news agency also reported shooting in the Taamir area which houses the Ain el-Helweh Palestinian refugee camp.
During Sunday's fighting, Assir distributed a video message by mobile phone to his supporters.
"We are being attacked by the Lebanese army," he said, describing the military as "sectarian" and accusing it of supporting Hezbollah chief Hassan Nasrallah.
"I call on everyone... to cut off roads and to all honourable soldiers, Sunni and non-Sunni, to quit the army immediately," Assir said.