BEIRUT — U.S.-backed Syrian fighters cleared explosives in the last area retaken from the Islamic State group on Sunday and arrested a number of militants hiding in tunnels, a day after declaring military victory and the end of the extremists’ self-styled caliphate.

The U.S.-led coalition said the clearing operations will continue until the area is secure.

A spokesman for the Kurdish-led Syrian Democratic Forces, who goes by the nom de guerre Mervan The Brave, said Baghouz village where the militants made their final stand is "full of all kinds of explosives." He said SDF forces have detonated land mines and suicide belts left behind by the militants.

The Kurdish Hawar News Agency reported that during the combing SDF forces arrested a number of militants found hiding in combat tunnels. Mervan The Brave confirmed the reports but had no further details.

Other activist groups monitoring the area reported limited clashes between remaining militants and SDF fighters.

In a series of tweets, the U.S.-led coalition said the SDF continued "back-clearance operations" to rid Baghouz of any militants or weapon caches.

“The Syrian Democratic Forces will continue to deny Daesh a physical space and influence in the area and work to deny them the resources they need to return,” it said using the Arabic acronym for ISIS. “This back-clearance operation will be deliberate and thorough and help ensure the long-term security for the area.”

A U.S.-backed Syrian Democratic Forces (SDF) fighter stands on a rooftop overlooking Baghouz, Syria, after the SDF declared the area free of Islamic State militants after months of fighting on March 23, 2019. (Maya Alleruzzo/AP)
A U.S.-backed Syrian Democratic Forces (SDF) fighter stands on a rooftop overlooking Baghouz, Syria, after the SDF declared the area free of Islamic State militants after months of fighting on March 23, 2019. (Maya Alleruzzo/AP)

A day earlier, a Syrian driver working with NBC News reporters was killed by an explosive device that went off in a house used by the SDF as a command post and a media center for journalists covering the fighting in Baghouz.

Noah Oppenheim, the president of NBC News, said in a statement that network employees escaped unharmed and that the reason for the explosion was being investigated. He expressed "deepest sympathies" to the driver's family and said the network is in touch with them to "support them however we can."

The victory announced in Baghouz on Saturday marks the end of a devastating five-year campaign by an array of forces to retake territories held by ISIS in Syria and Iraq. At its height, ISIS controlled a sprawling self-declared caliphate between the two countries that was home to some 8 million people.

The campaign against the group came at a staggering cost, with entire neighborhoods and towns destroyed in both countries. Thousands of civilians are believed to have been killed by ISIS during its brutal reign as well as the air and ground operations by the coalition and allied troops. Watchdog groups continue to document the casualties.

Meanwhile, the SDF said 11,000 of its fighters were killed in the fight against ISIS.

The militants were holding hostages and had detained civilians, the fate of many also remains unknown.

Despite the territorial defeat, the militants have reverted to insurgent tactics and continue to carry out attacks in areas liberated months or even years ago. An unknown number of fighters and supporters slipped into Syrian and Iraqi towns and villages, forming the backbone of the new insurgency.

The fate of the group's leader, Abu Bakr al-Baghdadi, remains unknown. There is a $25 million bounty on his head.

In recent weeks, thousands of people, including ISIS fighters and their family members, left Baghouz and were taken to detention centers and crowded camps for the displaced elsewhere in eastern Syria.

“This is an historic moment, but we cannot be complacent,” tweeted Maj. Gen. Christopher Ghika, the deputy commander of the U.S.-led coalition against ISIS.

“Even without territory, Daesh will continue to pose a threat to the people of Iraq and Syria, as well as to the wider world. The coalition must remain firm in its determination to counter Daesh,” he said.

Separately, Syrian state media reported that nearly two dozen people have suffered from asphyxiation after shells were lobbed from rebel-held areas into government-administered villages in the country's west. The state news agency quoted director of a local hospital in Hama province as saying that 21 people were hospitalized after inhaling gases in the attack on al-Rasif and al-Aziziyeh villages.

The pro-state Al-Ikhbariya TV interviewed survivors at the hospital who said a foul smell followed the launching of mortars into their villages.

The months-old truce, sponsored by Russia and Turkey, in Syria’s west has been tested, as fighting between government forces and al-Qaida-linked militants resumed. The government wants to regain control of a key highway that flanks the opposition-held area. Al-Qaida-linked militants have expanded their dominance in northwestern Syria, rooting out other armed opposition groups and undermining the truce.