DEPOK, Indonesia (Reuters) - Islamist militant prisoners have killed five members of an elite Indonesian counter-terrorism force at a high-security jail and were holding one of them hostage on Wednesday, police said.

A mobile brigade policeman patrols near an armoured vehicle at the Mobile Police Brigade (Brimob) headquarters in Depok, Indonesia, May 10, 2018. REUTERS/Darren Whiteside

One convicted terrorist had also been killed in clashes at the jail in Depok, on the outskirts of the capital Jakarta, national police spokesman Muhammad Iqbal told reporters.

Authorities were unsure how many people remained inside the prison complex.

“Five of our colleagues died ... one of our colleagues is still inside and being held hostage,” Iqbal said, adding that the hostage was reported to be safe.

It was the highest death toll suffered by the counter-terrorism police force, known as Detachment, or ‘Densus’, 88, which was set up in 2003 and received training and funding from Australia and the United States.

The squad largely broke up militant cells in the world’s largest Muslim-majority country. But a recent spate of low-level attacks inspired by Islamic State prompted authorities to more than double its force to 1,300 late last year.

Chief security minister Wiranto said the incident was being treated “very carefully because it concerns national security”.

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Police spokesman Iqbal said the prison, at the Mobile Police Brigade headquarters, held about 130 convicted terrorists, but it was unclear how many were involved in the clashes, which broke out late on Tuesday.

The prisoners are suspected of having taken weapons from the police who were killed.

Police said four officers were negotiating with the prisoners.

“If they surrender, it will be a non-violent process. But if there is a threat to officers or others, we will take very strong action,” Iqbal said.

In a message carried on its Amaq news agency, Islamic State claimed responsibility and said 10 counter-terrorism officers had been killed.

Iqbal denied that Islamic State was involved. He said a dispute had broken out over checks by prison authorities of prisoners’ food.

But another police spokesman, Setyo Wasisto, said some of the prisoners had recently met Aman Abdurrahman, who is believed to be the ideological leader of IS sympathizers in Indonesia. He is on trial for terrorism and is held at the same prison.

Police maintained tight security around the jail and kept onlookers away.

Islamic State sympathizers have carried out a series of mostly low-level attacks in Indonesia over the past few years.

Last year, an attack by two suicide bombers, using pressure cookers packed with explosives, killed three police and wounded 12 people at a Jakarta bus terminal.

In January 2016, four Islamic State-linked militants mounted a gun-and-bomb attack at an intersection outside a Starbucks cafe in central Jakarta, killing themselves and four others.