A Lebanese soldier and four attackers were killed in two separate assaults late Sunday on army checkpoints in the southern city of Sidon, the military said in a statement.

"At 9:15 p.m. (1915 GMT) today (Sunday) an armed man approached an army checkpoint in the north of Sidon, and launched a hand grenade towards it, injuring two soldiers," the army statement said.

"Troops manning the checkpoint fired back at the attacker, leading to his death," it added.

Then at 10:20 p.m. (2020 GMT), three armed men in a four-by-four vehicle approached a second army checkpoint at another location in the southern city.

One of them "blew himself up with a hand grenade he was holding, killing him and a soldier," said the army, adding that another soldier was wounded in the attack.

"Then troops at the checkpoint opened fire at the other armed men and killed them," the military added.

The army gave no details on the identities or nationalities of the attackers, but said the military police has opened an investigation into the incidents.

A security source in Sidon, southern Lebanon's largest city, meanwhile told AFP on condition of anonymity that "the attacks were likely coordinated."

AFP's correspondent in Sidon said the army deployed vehicles along the city's main roads as it stepped up security.

Sidon is located some 40 kilometres (25 miles) south of Beirut.

The majority Sunni city was the scene of fierce battles in June that pitted the army against fighters loyal to a radical cleric, Ahmad al-Assir, who has since been on the run.

Assir is fiercely opposed to the powerful Shiite movement Hezbollah and its ally in Damascus, President Bashar al-Assad.

The June battle claimed the lives of 18 Lebanese troops and 11 fighters, and ended when the army seized control of Assir's compound in the city.