WASHINGTON- President Barack Obama will highlight inequalities in the US tax system Tuesday, by welcoming the secretary of billionaire financier Warren Buffett to his State of the Union address.

The secretary, Debbie Bosanek, has become a fixture in Obama's campaign rhetoric after her boss complained that a tax system which required his staff to pay at a higher rate than he does, cannot be fair.

Bosanek will be a guest of honor in First Lady Michelle Obama's box in the House of Representatives when Obama starts his annual showpiece speech at 9:00 pm (0100 GMT), senior Obama aide Dan Pfeiffer said on Twitter.

The invitation to Bosanek was the latest sign that Obama will take another swing at raising taxes on the wealthiest Americans to finance jobs programs for the middle classes and to enhance US competitiveness.

"Warren Buffett's secretary shouldn't pay a higher tax rate than Warren Buffett. There's no justification for it," said Obama in September.

The president appears set to make a new push for Congress to adopt the so-called "Buffett Rule" which was proposed by the legendary investor and would see millionaires pay a new, higher tax rate.

Currently, investment income is taxed at a much lower rate, sometimes around 15 percent, than other types of income, meaning that many rich Americans effectively pay a lower tax rate than the less well off.

Republicans have resisted higher taxes on wealthy Americans, saying such a move would crimp small businesses which act as an engine of job creation.

In a landmark article in the New York Times last August, Buffett complained that the tax he paid on his vast wealth came in at 17.4 percent, lower than 20 other people who worked in his office.

He said it was time for the government to close loopholes which benefit the super rich, at a time of high deficits and economic blight.

Obama's call for the Buffett rule also comes at a time when Mitt Romney, the man many observers view as his most likely Republican foe in November's election, is also under fire for his tax practices.

Romney reluctantly released recent tax returns on Tuesday which showed he earned $21.7 million in 2010 -- and was taxed at 13.9 percent -- a rate much lower than that of average Americans.

Obama is using the State of the Union address to reinforce his campaign narrative that Republicans are guilty of tilting the economy in favor of the rich and are depriving hard-pressed middle classes of a "fair shake."

Bosanek will be joined in Michelle Obama's box on Tuesday by Mark Kelly, the shuttle astronaut husband of shot lawmaker Gabrielle Giffords, who has announced she will step down to concentrate on her recovery.

Giffords, an Arizona Democrat, was shot by a deranged gunman last year in an assassination attempt in which six people, including a nine-year-old girl were killed.

The tradition of welcoming national heroes to the State of the Union address dates back to former president Ronald Reagan, who honored Lenny Skutnik after he dived into an icy river to save a woman passenger of an airliner which crashed shortly after takeoff from Washington's National Airport in 1982.