Billionaire software baron turned philanthropist Bill Gates has warned that violence in Nigeria and Pakistan could set back his goal of eradicating polio by 2018.

Last year, 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 on Tuesday that major challenges remain.

India, which once had the world's worst polio -- a mainly childhood disease that causes the wasting of the limbs -- has just celebrated three years free of the disease.

But it 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, complaining that local conspiracy theories have undermined inoculation 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 of 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.

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

The 58-year-old Harvard dropout with a net worth of more than $70 billion, and who promises to give away all his money within 20 years of the death of either him or his wife, is an optimist.

On Tuesday his Foundation published its annual letter disputing three myths that hinder progress: that poor countries stay poor, foreign aid is pointless and saving lives inflates populations.

By 2035 he believes there will be almost no poor countries left, singling out China, Brazil and India as "wonderful examples" of states that now have high numbers of middle-income earners.

And as he battles to reverse lackluster educational standards within the United States, he said lessons could be learned from China.

"Anyone who thinks the world was better off when China was poor, that's very anti-humanitarian," he said.

"The fact they run a good education system, yes we should all go and learn from that," Gates added.

"China comes up with cancer medicines? I won't hesitate to have my kids or anyone benefit from that. This is not a zero sum game... The uplifting of China can be overwhelmingly good news."

According to a recent survey conducted by YouGov, Gates is the most admired person in the world, but asked how that makes him feel, he was temporarily lost for words.

"If philanthropy is getting more popular that's a good thing. If entrepreneurship is more popular, that's a good thing. If people want to give to helping the poorest that's a good thing."