Jailed Indonesian militant cleric Abu Bakar Bashir has pledged allegiance to jihadists who have overrun swathes of Iraq and claimed leadership of the world's Muslims, a radical group said Tuesday.

The news came as authorities in Indonesia, the world's most populous Muslim-majority country, announced measures aimed at curbing growing support for the Islamic State (IS) group, including the blocking of websites that support them.

Indonesia has for years struggled with Islamist radical groups and has been hit by numerous militant attacks, including the 2002 Bali bombings that killed 202 people, mostly foreign tourists.

A successful crackdown over the past decade has weakened many groups but authorities believe dozens of Indonesian radicals have headed to fight in Syria and Iraq, and fear they could revive networks on their return.

Authorities had already voiced suspicions that Bashir, regarded as the spiritual leader of militant Islam in Indonesia, was funding IS, which in June declared an "Islamic caliphate" in territory it controls across Iraq and Syria.

On Tuesday Jemaah Anshorut Tauhid (JAT), a radical group founded by Bashir, confirmed that the frail, bespectacled cleric had sworn allegiance to IS in a maximum-security prison on an island off Indonesia's Java several weeks ago.

The 75-year-old "has led a pledge of allegiance with other prisoners soon after the declaration of the new caliph," Afief Abdul Madjid, the group's acting leader, told AFP, adding the cleric had urged his followers to do the same.

Reports said the ceremony to swear allegiance took place in a prayer room.

Bashir, a vocal supporter of Al-Qaeda style jihad, has been in and out of prison for years and is currently serving a 15-year term for funding terrorism.

Authorities on Monday announced the move to block websites and said efforts would be made to stop would-be jihadists heading to the Middle East, including by working with religious figures to highlight the dangers.

"Any efforts to spread (IS's) teaching must be prevented," said Security Minister Djoko Suyanto.

Bashir was one of the alleged founders and chief ideologue of regional terror network Jemaah Islamiyah, which was blamed for the Bali bombings.

There have been no major terror attacks in Indonesia for several years. Recent attacks have mostly been low-level and have targeted domestic security forces.