DENPASAR (Reuters) - Indonesian authorities on the resort island of Bali on Thursday (Sep 22) detained a ship from Malaysia carrying around 30 tonnes of fertiliser which police believe may have been intended for making bombs.

Customs and police were questioning the crew and investigating the material for potential links to radical networks as the world's largest Muslim-majority nation remains on high alert for militant attacks.

Bali police official Hendra Suhartiyono said authorities were looking into whether the material was on its way to the eastern island of Sulawesi, a region known for militant violence. "We are not closed to the possibility that this chemical material ... could also be for the benefit of terrorist groups in Sulawesi to make low-impact and high-impact bombs," he said.

Indonesia's most-wanted man, a militant called Santoso, was killed by security forces in Sulawesi earlier this year.

Santoso, who had pledged allegiance to Islamic State, cultivated a small radical network in the Poso area, which has now been severely weakened by a lengthy security operation. "At the moment the crew are being intensively examined on explosive material ammonium nitrate that was carried, shipped from Malaysia," Bali customs official Thomas Aquino said.

"They confessed that the boat was rented to be shipped to Sulawesi. They thought the material in the sacks was fertiliser. We will detain the ship crew to be processed legally."

Indonesia saw its first militant attack in several years in January in which four people were killed. The gun-and-bomb assault in the heart of the capital Jakarta was claimed by Islamic State.

Last month, authorities tightened security in Bali after reports of a suspected militant plot on the island.

A nightclub bombing on the island in 2002 carried out by home-grown militants killed 202 people, mostly Australians, and prompted a nationwide security crackdown.