The powerful boss of one of Malaysia's poorest states has used graft and abuse of power to amass a staggering $15 billion fortune -- making him the country's richest man -- an activist group said.

The Swiss-based Bruno Manser Fund (BMF) released a report this week alleging that Abdul Taib Mahmud, the chief minister of resource-rich Sarawak state since 1981, and 20 family members are collectively worth $21 billion.

The vast wealth has been accrued through Taib's grip on huge swathes of the economy and rampant nepotism in handing lucrative contracts and concessions to family members and associates, it said.

In the key timber sector alone, Taib, 76, has "ultimate control over the granting of timber concessions" worth several billion dollars.

The family's wealth is allegedly spread over 400 companies around the globe, said the group, which called for an international investigation into the alleged ill-gotten gains.

"Taib has abused his position... to award his family members vast timber concessions, palm oil concessions, state contracts and directorships in Sarawak's largest companies," said the group, which campaigns for rainforest protection and is a fierce critic of Taib.

According to Forbes magazine, 88-year-old tycoon Robert Kuok is Malaysia's richest man, with assets worth $12.4 billion as of March.

Taib opponents have long alleged systematic corruption, abuse of power, nepotism, and plundering of the rich natural resources of Sarawak, located on the northern portion of Borneo island.

For years, Malaysia's anti-corruption agency failed to move against Taib. But, under pressure over the mounting allegations, said this year it had opened an investigation but has provided no details.

Critics of Taib and Malaysia's long ruling party accuse the federal government of failing to act because his tight control of Sarawak has kept it a vital ruling party stronghold.

An official in Taib's office could not be reached for comment. Taib has previously denied such charges.

The BMF said Taib conceals his wealth by rarely being listed as a director or shareholder, instead appointing nominees, usually family members, to act for him.

The group said it arrived at the wealth estimate by reviewing financial and corporate records in Malaysia and several other countries, and a range of other sources.

In December, the group spearheaded a campaign with 16 other environmental groups and activists including Greenpeace calling for the arrest of Taib, Malaysia's longest-serving state chief minister.

His opponents allege massive graft in awarding Sarawak timber concessions and other contracts, and rapacious development that has seen rainforests felled, questionable dams built, and tribal groups uprooted.

Taib's government has defended its policies, saying the state needs to be developed.