Sunni Muslim gunmen, hold up their weapons Sunday as they roam the streets, expressing solidarity with Salafist leader Ahmad al-Assir in Tripoli. (Omar Ibrahim/Reuters)

The army’s death toll in a tense stand-off with Sunni gunmen in the southern Lebanese city of Sidon had climbed to 12 by Monday morning, as violence triggered by the war in Syria engulfed its smaller neighbor.

The army said it was the subject of a “cold-blooded” attack by armed gunmen intent on plunging Lebanon back into the strife of its own 15-year civil war. Sunni militiamen loyal to outspoken Sidon cleric Sheik Ahmed al-Assir fired rocket-propelled grenades at Lebanese forces during the standoff, according to the army and the National News Agency.

The war in Syria has polarized Lebanon, straining relations between Sunnis and Shiites. The situation has been further inflamed by the decision of the powerful Lebanese Shiite militant movement Hezbollah to send fighters to back President Bashar al-Assad’s forces in Syria.

It was the second time in less than a week that clashes have erupted in Sidon, a coastal city 25 miles south of the capital, signaling Lebanon’s growing inability to insulate itself from the Syrian war.

Sunday’s fighting marked a dangerous escalation as the army, seen by many as the glue that holds the country together and an institution that stands above sectarian divisions, became the target, with Assir lambasting it for perceived links to Hezbollah.

As the night wore on and fighting intensified, army checkpoints also came under fire in the northern city of Tripoli and next to the southern Palestinian refugee camp of Ain Helweh. Roads were temporarily blocked by Assir’s supporters up and down the coast and in the capital.

Assir, a vehement critic of Hezbollah, has clashed with the Shiite movement’s supporters in Sidon in the past, but the intense exchange with the army Sunday took the confrontation to a new level.

“What happened today in Sidon went beyond all expectations,” the army said in a harshly worded statement. “The army was targeted in a cold-blooded and deliberate attack.”

It said 12 soldiers had been killed in the attack, with 50 others injured. The statement evoked comparisons to Lebanon in 1975, the year of a fishermen’s strike in Sidon that is considered by many to have been a key incident in setting the fuse for civil war.

The army said Sunday’s fighting began when supporters of Assir launched an unprovoked attack on one of its checkpoints in the city’s Abra suburb. Assir said in a statement that soldiers at the checkpoint had beaten up two of his supporters before opening fire on others who went to their assistance. The army later besieged his mosque with heavy artillery, he said.

The clashes caused panic in the normally sleepy southern city, sending residents fleeing for safety. The army had surrounded Assir’s mosque in the early hours Monday morning, with the cleric believed to be holed up inside. Two Sunni gunmen also were killed in the fighting, according to Reuters.

Assir posted a video online Sunday saying he was under attack from the army and called for Sunni soldiers to defect. He lambasted the armed forces as working at the behest of the Iranians and Hezbollah and urged Sunnis to block roads in solidarity.

Lebanese President Michel Suleiman called for an emergency meeting with ministers and security officials Monday to discuss how to contain the violence. He said Assir’s calls for “jihad” against the military served only Lebanon’s enemies.

Heavy military deployments could be seen in the capital amid fears the violence would spread. The road from Beirut to Sidon was blocked by burning tires for several hours, while gunmen also blocked roads in the northern city of Tripoli, where an army base came under attack from rocket-propelled-grenade fire. Roads were also blocked in the capital, where clashes briefly broke out in Beirut’s southern suburbs, according to Future TV.

Issam Majzoub, a Sunni leader in Masnaa, in the eastern Bekaa Valley, said he was gathering supporters to close the main crossing into Syria. “Either the army is for everyone or we will have our own army,” he said.

Across the border in Syria, the cycle of violence continued Sunday with a spate of bomb attacks. Three suicide bombers targeted a security building in the Rukn al-Din area of the capital, Damascus, killing five people, according to official state media. Meanwhile, 12 died in a bomb blast in the northern city of Aleppo, according to the Britain-based Syrian Observatory for Human Rights.

Ahmed Ramadan and Suzan Haidamous contributed to this report.