WASHINGTON — US mega-billionaire Warren Buffett challenged other wealthy Americans, including News Corp magnate Rupert Murdoch, to bare their income and tax details when he revealed his own in a letter.

In the letter to a congressman, Buffett, the world's third richest man, said he earned a total of $62.8 million last year, which when adjusted left $39.8 million in taxable income.

Buffett, who backs President Barack Obama's push for wealthy Americans to pay more in taxes, said he paid a total of $6.9 million in tax on the final number -- an effective 17.3 percent rate.

That was a far lower rate than what many Americans, including his own secretary, pay, Buffett said told Republican Representative Tim Huelskamp in the letter, dated Tuesday.

Buffett told Huelskamp, a critic of his tax views, that he reckoned that most senators and representatives pay more than 30 percent of their incomes in taxes, "just like all of the people in our office except for me."

Buffett, who built a $50 billion fortune on investments through his Berkshire Hathaway company, was answering Huelskamp's call to prove his low tax claims by publishing his complete tax form.

Buffett fended off that, saying it would prove little, by itself.

"What would be useful would be to get more of the ultra rich to publish their returns," Buffett wrote. "If you could get other ultra rich Americans to publish their returns along with mine, that would be very useful to the tax dialogue an intelligent reform," he said.

"Clearly, many of the ultra rich are paying even lower percentages than I do to the federal government, and obtaining their returns should be useful to our legislators in crafting an equitable tax code."

He included in his call specifically "Rupert Murdoch, one of my ultra-rich colleagues", after Murdoch's wall Street Journal last week blasted Buffett's advocacy of higher taxes for the wealthy and said he should release his tax forms.

Huelskamp replied on his website that he was disappointed that Buffett still hasn't released his official tax documents.

Moreover, he added, "If Mr. Buffett is truly concerned about paying his 'fair share' to the government, he is more than able to send voluntary contributions to President Obama and the rest of the federal government. Apparently he has not done so."