Another gunman fled to an unknown location, according to an army statement.
The attack struck an army post in the Minyeh region, about 90 kilometers (55 miles) north of Beirut An army operation was underway to arrest the fugitive and uncover the circumstances of the attack, the military said.
The attack occurred as the army was elsewhere engaged in a heavy exchange of fire with a group of militants linked to Sunni extremists. The standoff in northeast Lebanon, close to the border with Syria, began after Lebanese forces raided a house where the group was holed up.
Videos circulating online from the remote Wadi Khaled area showed the use of rocket propelled grenades in the battle. The Lebanese army sealed off the area and had not commented on the operation.
A security official said the group includes militants linked to one of Lebanon's most wanted extremists, Khaled Tellawi, who was killed earlier this month during a raid in the country's north that also left four Lebanese soldiers dead.
Tellawi was blamed for an attack last month that killed three men in a predominantly Christian northern village. He was described as a member of an extremist group that had links to the Islamic State group. The security official spoke on condition of anonymity in order to discuss the operation.
Separately, the United Nations peacekeeping force in south Lebanon deployed a detachment of multinational forces to Beirut on Sunday to assist Lebanese authorities with efforts to deal with the aftermath of the Aug. 4 port explosion.
The catastrophic explosion, due to the detonation of nearly 3,000 tons of ammonium nitrates that had been stored for years in a port warehouse, decimated the country's main commercial facility and killed 193 people.
The deployment follows a request from the Lebanese military and authorization by the U.N. Security Council for the Mission to take “temporary and special measures” to provide support to Lebanon and its people in the aftermath of the blast.
UNIFIL peacekeepers deployed to the Lebanese capital with heavy machinery and other equipment, a statement said, adding that the main areas of support will be clearing of debris and construction work in order to facilitate the rapid resumption of operations in the Beirut harbor.
“It is important for a Mission like UNIFIL with over 10,000 troops to help the country that has hosted us for more than 42 years,” said UNIFIL Head of Mission and Force Commander Maj. Gen. Stefano Del Col.
UNIFIL was originally created to oversee the withdrawal of Israeli troops after a 1978 invasion. The mission was expanded in Lebanon under a U.N.-brokered cease-fire following a monthlong war between Israel and Hezbollah militants in 2006.