BEIRUT — Two senior Islamic State group fighters, including a leader of the extremist organization, were killed in Lebanon Thursday, as the country’s army carried out an operation targeting one of the militant group’s “key posts” on the nation’s northeastern border. A third man was arrested.

Nayef Shaalan, who also was called Abu Fouz, was the leader of the extremist group aka either ISIL or ISIS in the border town of Arsal, long a stronghold of the militant group. During clashes with the Lebanese army on the outskirts of Arsal, Shaalan and his “Syrian escort” Ahmad Mroueh were killed, while ISIS’ regional security official, a Syrian national named Moustafa Mousalli, was detained, the army said in a statement.

Lebanese security officials previously had been monitoring Shaalan’s whereabouts, accusing him of either carrying out or planning several attacks on the Lebanese side of the border with Syria. This week, Hezbollah said it thwarted an ISIS attack on one of its bases near Arsal.

“The targeted terrorists had participated in fighting against the army in 2014, and they are responsible for rigging a number of cars and causing several explosions that targeted army posts and civilians in Arsal and its surroundings,” the Lebanese army said in its statement.

Arsal has been a hotbed of fighters from both al Qaeda’s Nusra Front and ISIS for almost two years, since the groups coordinated on the kidnapping of 30 members of Lebanese security forces in the town in August 2014. The extremists have employed the captives as leverage to buy themselves freedom of movement in Arsal and its surrounding mountainous region.

nusra lebanon Al Qaeda-linked Nusra Front fighters carry their weapons on the back of a pickup truck during the release of Lebanese police and soldiers in Arsal in the eastern Bekaa Valley of Lebanon Dec. 1, 2015. Photo: Reuters/Stringer

Most of that time, Lebanese security forces and Hezbollah, the armed Shiite Muslim party in the country, were able to carry out only minimal operations around Arsal because of fears hostages would be killed in retaliation.

Late last year, however, the Lebanese army and Hezbollah stepped up attacks in the border region after the Nusra Front agreed to a prisoner swap. Meanwhile, ISIS still holds hostage nine members of the country’s security forces.