Indonesian forces are on high alert for reprisal attacks after the country's most-wanted militant was killed this week, officials said on Wednesday (July 20).

Police confirmed Santoso, among the first Indonesians to pledge loyalty to Islamic State, was killed in a gun battle with security forces on the island of Sulawesi on Monday (July 18).

But officials say the threat level in the world's largest Muslim-majority nation remains high.

Rudy Sufahriadi, the police chief for central Sulawesi, said the security operation in Poso, where the U.S.-designated "terrorist" Santoso had been hiding, would continue adding there was a possibility of a backlash.

Chief Security Minister Luhut Pandjaitan told reporters in Jakarta operations would be intensified in regions considered hotbeds for radicalism.

Around 20 members of Santoso's Mujahidin Indonesia Timur remain in hiding in the jungles of Sulawesi, where Indonesia has been running a major security operation for years.

"Of course we are pursuing the rest of Santoso's people over there, I think still another 19, but in the same time we are conducting right now the territorial operation over there: how to brief people, how to convince people that situation is under control," he said.

A team of police and military personnel also shot a man believed to be Santoso's right-hand man on Monday, a move officials say will weaken the group.

President Joko Widodo last year stepped up efforts to capture or kill Santoso, ordering the military to support thousands of police scouring the jungles where he was hiding. The effort included fighter jets and warships.

The majority of Indonesians are moderate Muslims but militants have launched sporadic attacks over the past 15 years.