Iraq's justice minister has accused the US of concealing information about deposed president Saddam Hussein that could be damaging to "many countries".
Saddam Hussein was captured in December 2003
Abdel Hussein Shandal said it seemed there were "lots of secrets" that the Americans wanted to hide.
Saddam Hussein is set to go on trial in Iraq over alleged crimes against humanity, but no date has been set.
Mr Shandal spoke to the AP news agency on the eve of a major conference. US officials did not immediately comment.
"There should be transparency and there should be frankness, but there are secrets that, if revealed, won't be in the interest of many countries," Mr Shandal said.
"Who was helping Saddam all those years?"
Saddam Hussein is officially in Iraqi custody but is being guarded by US troops at a secret location.
The minister expressed his confidence that the investigation into Saddam Hussein would be finished by the end of the year and that this would be followed by a speedy trial.
But an official from the Iraqi tribunal set up to try the former president told AP that no date had been set.
"The interrogation of Saddam is taking place regularly and almost daily and neither the justice minister, not the Americans, have anything to do with it because the tribunal is an independent court," the official said.
US officials are said to have privately warned against rushing into a trial until Iraq has a satisfactory judicial system - one of the main topics to be discussed at the Brussels conference.
The US is also believed to be worried about holding a trial before a new constitution is written and while there is still so much violence.
The international conference on Iraq's future is to take place over two days and will be co-hosted by the European Union and Washington.
UN Secretary General Kofi Annan and US Secretary of State Condoleezza Rice will be joined some 80 foreign ministers and officials.
Iraq will appeal to donor countries to fulfil their financial pledges and to pursue the question of debt relief.
They are also hoping for extra support to train and equip Iraqi security forces, as well as help with judicial and penal reforms.