The US Secretary of State has said she is unwilling to answer a House of Representatives subpoena about Iraq's alleged pre-war weapons.

The House is looking to question Ms Rice about a White House assertion that Iraq was trying to buy uranium from Niger to build nuclear weapons. The claim which has since been discredited, was used as U.S. justification for invading Iraq in 2003.

Rice claims that her staff has already written three letters in the last month to Democratic Congressman Harry Waxman, chair of the House oversight committee about the matter.

She says that as President Bush's national security advisor at the time, she was shielded by the constitutional principle of executive privilege.

"There is a separation of powers, and advisors to the president are, under that constitutional principle, not generally required to go and testify in Congress," she told reporters in Oslo during a meeting of Nato foreign ministers.

Mr Waxman however says the committee has "hit a brick wall with the secretary of state".

"She will not propose to testify, she will not agree to testify, and she insists our committee be satisfied with partial information that was previously submitted to other committees."

Last night he said that Ms Rice had left his committee with "no choice but to proceed with the subpoena".

The committee voted 21-10 to issue a subpoena to compel Ms Rice to testify.

SAS/HAR