ABUJA, March 8 (Reuters) - A Nigerian human rights activist has been attacked by thugs and received death threats after accusing government officials in the nation's top oil-producing state of corruption, he said on Thursday.

Anyakwee Nsirimovu, director of the Institute for Human Rights and Humanitarian Law, said he had received a stream of threatening phone calls over the past two weeks and was lucky to escape with his life after an attack on his car on Sunday. "I am being attacked because we have accused the Niger Delta governors of stealing public money and driving conflict in the region," he told Reuters.

U.S. based Human Rights Watch called on the Nigerian government to investigate and prosecute those responsible for the assault and death threats.

"These attacks threaten free speech on key issues like governance and corruption during the critical pre-election period," said Peter Takirambudde, the group's Africa director.

Nigerians go to the polls next month to elect a new president, state governors and lawmakers, marking the country's first handover from one elected civilian government to another. Nsirimovu said he was taking advantage of a move against some corrupt politicians by the federal government to publicise alleged looting of public resources in the oil producing region.

Nsirimovu issued a scathing public criticism of Rivers State lawmakers when they dropped demands that Governor Peter Odili account for $1.3 billion spent in 2006, as required by law, before submitting the 2007 budget to a vote.

Rivers State officials deny any wrongdoing.

Anonymous callers told Nsirimovu to stop publicising corruption allegations against government officials in Rivers and told him he was not safe.

"It might get worse as the elections approach because I think the objective is to shut people up," Nsirimovu said.

Human Rights Watch said in a report in January that local governments in Rivers routinely stole and misused public funds resulting in a disastrous failure to provide basic services and aggravating social unrest.

Rivers is at the epicentre of a collapse in law and order in the Niger Delta, which is home to all of Nigeria's 2.5 million barrels per day of oil production.

Thousands of foreign oil workers have already fled the state after a wave of kidnappings and attacks, sometimes by ransom-seekers and sometimes by militant groups seeking more regional control over the delta's oil riches.

Nsirimovu said government officials were arming gangs in the region and violence would worsen unless the political class could produce better leaders.

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