NEW YORK: Billionaire software baron turned philanthropist Bill Gates warned on Tuesday that violence in Nigeria and Pakistan could set him back in his goal of eradicating polio by 2018.

In 2013, The Bill and Melinda Gates Foundation, a charity that funds medical research and vaccination drives -- made wiping out the crippling disease in the next six years its top priority.

But the Microsoft founder, who has poured a large part of his personal fortune into the drive and encouraged fellow billionaires to contribute, said in an AFP interview that major challenges remain.

India, once the country with the worst problem with polio, a mainly childhood disease that causes the wasting of the limbs, has just celebrated three years polio free.

But the disease remains endemic in Afghanistan, Nigeria and Pakistan. There are also reinfections in war-torn Somalia and Syria that threaten to break out into areas once free of the scourge.

“Nigeria and Pakistan are going to be tough. The Pakistan violence is evil,” Gates told AFP in New York on Tuesday, complaining that local conspiracy theories have undermined innoculation drives.

“The truth is the vaccine is to help kids. And spreading rumors and attacking the workers on this, those people don't have justice and truth on their side.

“And so we may miss by a year or two if we can't help out with that. The president, the religious leaders a lot of the supporters of that country are trying to get the truth out.”Just hours before Gates spoke, three polio workers were shot dead in the Pakistani city of Karachi, forcing the suspension vaccination in the whole southern province of Sindh.

Last week the World Health Organization warned that Pakistan's northwestern city of Peshawar was the world's “largest reservoir” of the disease.

Opposition from the Pakistani Taliban to immunization and an Islamist insurgency in northern Nigeria have also hit hard. “This is really going to come down to Nigeria and Pakistan,” Gates told AFP.

“Everyday we're talking about what's going well, what's not, how we track the teams, where new approaches can help out so we've intensified the effort,”he added.

Last November the Global Polio Eradication Initiative said Nigeria had 51 of the 328 cases of the disease worldwide in 2013, compared to 121 out of 223 in 2012.

But numbers are up in Pakistan. According to the WHO, Pakistan recorded 91 cases of polio last year compared with 58 in 2012.

“Even in Pakistan it's somewhat of an increase but still small numbers so we're very close,” Gates told AFP. The Gates Foundation spends more than $300 million a year on polio programs and US Congress has also allocated extra money.

“I think we'll have the money. I think we've got the will. We need, on the ground, to get the truth out,” Gates said.