(Bloomberg) -- Elon Musk, the world's richest man, challenged a United Nations official's claim that just a small percentage of his wealth could help solve world hunger.

Musk was responding to comments by David Beasley, director of the UN's World Food Programme, who repeated a call last week following an earlier tweet this month asking billionaires like Musk to "step up now, on a one-time basis."

Beasley specifically called for action from Musk and Amazon.com Inc. co-founder Jeff Bezos, the two men atop the Bloomberg Billionaires Index.

Just $6 billion could keep 42 million people from dying, Beasley said.

If the World Food Programme, using transparent and open accounting, "can describe on this Twitter thread exactly how $6B will solve world hunger, I will sell Tesla stock right now and do it," Musk wrote in a Twitter post.