The death toll in clashes in Lebanon’s southern city of Sidon continued to climb Monday, marking the bloodiest 48 hours for the country’s army since the conflict in Syria began creeping over the border and evoking memories of Lebanon’s own 15-year civil war.

Fifteen soldiers have been killed in the fighting between the military and Sunni militia supporting Sidon’s firebrand cleric Sheik Ahmed al-Assir, according to a senior security official. Brig. Ali Shahrour said the area around Assir’s mosque, the epicenter of the fighting, had been secured, while the army was hunting for the cleric to arrest him.

While Lebanon has sporadically erupted in sectarian clashes over the past two years, the fighting in Sidon provides a significant test for the country’s security forces, with the potential to pull the Lebanese army into the sectarian fray as the regional Sunni-Shiite tensions grow in the fallout from Syria’s war.

In the latest bout of fighting, the army, seen as an important stabilizing force in a country that is suffering from a political vacuum, risks being seen as taking sides amid reports that militants from the Shiite movement Hezbollah are coordinating with the military, analysts said. In the northern city of Tripoli, members of a Sunni militia backing Assir have attacked an army position, and masked gunmen patrolling the streets have forced shops to close. One parliamentarian called for a curfew in cities, including the capital of Beirut, to contain unrest.

Assir is an outspoken critic of Hezbollah and has vehemently denounced the movement’s decision to send fighters to Syria. On Sunday, he accused the army of being a proxy of Hezbollah, and his followers fought pitched battles against the army with rocket-propelled grenades and machine guns.

The location of the violence is particularly evocative — it was in Sidon in 1975 that the first sparks of the civil war were ignited with the assassination of Maarouf Saad, a prominent politician, at a demonstration. His son, Osama Saad, who has taken over his father’s mantle as leader of the Popular Nasserite Party, said events in Sidon over the past two days have brought back memories of that time.

“Today I am reminded of the atmosphere of the civil war . . . the shelling and the snipers,” he said in an interview with journalists. “For a long time we’ve warned that we might reach this point. All the Lebanese people remember the agony and struggle of the Lebanese civil war, and they are afraid.

However, he expressed optimism that the country can bounce back, with “lessons learned” by Lebanese politicians about the high cost of all-out war.

The exchange of heavy weaponry fire had abated by Monday afternoon. But with sniper fire continuing, the streets of this sleepy coastal town, just over 25 miles south of the capital, were deserted as many residents fled or holed up inside. Shahrour, head commander of the internal security forces for southern Lebanon, said the situation was under control.

Reports on the number of Assir’s supporters killed varied widely, with the National News Agency reporting that two had died and Shahrour putting the number at 40. Civilian casualties had not been counted but were limited, he said.

The last time the army suffered such a high death toll in such a short time frame was during the 2007 conflict in the Nahr al-Bared Palestinian refugee camp. The clashes in Sidon also risk stirring tensions in the camps, and gunmen from the group Fatah al-Islam, which fought against the army in the Nahr al-Bared conflict, joined Assir’s men in Sidon’s fighting, according to Shahrour.

He denied that Hezbollah was involved in the fighting, but one person close to the movement said four of its militants had been killed.

“This could make the army appear more like it’s on a side in this conflict,” said Imad Salamey, a professor in political science at the Lebanese American University. “The army is one of the last functioning institutions of the state.”

The army has threatened to strike with an “iron fist” against those that spill its blood but denied taking sides.

“The Lebanese army is united and has not assaulted any specific sect,” said the army chief, Gen. Jean Kahwaji, in comments carried by the National News Agency. “The army responded to an armed group that intentionally and purposefully attacked it.”

As the fighting lulled Monday, residents fled the city, fearful that there was more violence to follow. On a minibus at Sidon’s central bus station, a family of Syrian refugees was leaving for the capital.

“We don’t have a place to go, but we are worried,” said Mohammed Laymouni, 40. The family had fled Damascus two months earlier for the relative safety of Lebanon.

Suzan Haidamous and Ahmed Ramadan contributed to this report.