US President Barack Obama's camp accused Republican Mitt Romney of hiding his finances in a "black box" following a report he used an ethics loophole to avoid offering details of some earnings.

The Washington Post report unleashed a new spat between the president's campaign aides and Romney, the most likely Republican nominee, reflecting the quickening pace of the race towards November's election.

The report said that the multimillionaire and former Massachusetts governor legally used an obscure exception in ethics laws to provide a limited accounting of his assets, making it hard to trace exactly where his wealth is invested.

The paper said the strategy made it tough to determine whether there were any conflicts of interest, or if Romney's assets were held offshore or invested in controversial companies.

Obama's campaign manager Jim Messina complained that new questions were emerging every week about whether "Romney took unusual steps to avoid paying his fair share in taxes."

"Today's report suggests that governor Romney is exploiting a loophole in order to shield his assets and investments from public review," Messina said.

"Mitt Romney has put his personal financial assets in a black box and hid the key, attempting to play by a different set of rules than any candidate in recent history."

But Romney's campaign hit back, accusing the Obama team of trying to deflect attention from the president's own economic record.

"President Obama will do anything to try and distract Americans from his failed record of chronic unemployment, lower incomes and higher gas prices," said Romney spokeswoman Andrea Saul.

Saul said Romney's personal financial disclosure "completely and accurately describes governor Romney's assets as required by the law applicable to candidates for president."

She said government ethics monitors would have made clear if they found Romney's financial disclosures to be insufficient.

The Post said that Romney declined to identify underlying assets held in 48 accounts from Bain Capital, the private equity firm with which he built his fortune.

Romney was not required to reveal details of the assets concerned because they were covered by a confidentiality agreement with Bain, which is exempted from disclosure rules by the Office of Government Ethics.

The Republican candidate rode out a previous row over his financial and tax arrangements earlier in the nominating race, in which he is now the prohibitive favorite.

But the Obama campaign offensive Thursday signaled that the president's team will seek to revive the controversy, as it seeks to portray the wealthy Romney as indifferent to the economic struggles of ordinary Americans.