The US Secretary of State Condoleezza Rice has said she is unwilling to answer a US House of Representatives subpoena about Iraq's pre-war weapons.
Ms Rice was national security adviser before the Iraq invasion
A House committee wants to question Ms Rice about a White House assertion that Iraq was trying to buy uranium from Niger to build nuclear weapons.
The White House claim, now discredited, formed part of the US justification for invading Iraq in 2003.
Ms Rice said supplying written answers was the best way to pursue the issue.
She said that as President George W Bush's national security adviser at the time, she was shielded by the constitutional principle of executive privilege.
"There is a separation of powers, and advisers to the president are, under that constitutional principle, not generally required to go and testify in Congress," she told reporters in Oslo, Norway, during a meeting of Nato foreign ministers.
She said her staff had already written three letters in the last month to Democratic Congressman Henry Waxman, chair of the House Oversight Committee about the matter.
"If there are further questions that Congressman Waxman has, then I am more that happy to answer them again in a letter, because I think that that is the best way to continue this dialogue," Ms Rice said.
On Wednesday night, Mr Waxman said Ms Rice had left his committee with "no choice but to proceed with the subpoena".
"We have hit a brick wall with the secretary of state," he said as the Democratic-led committee voted 21-10 to issue a subpoena to compel Ms Rice to testify.
"She will not propose a date to testify, she will not agree to testify, and she insists that our committee be satisfied with partial information that was previously submitted to other committees."
He said Ms Rice's position as top security adviser to the president before the invasion of Iraq gave her unique insight as to why the White House had put forth its claim that Saddam Hussein's Iraq was buying uranium from Niger.
The CIA sent former diplomat Joseph Wilson to Niger to investigate the claim, which he said was unfounded.
His wife, Valerie Plame, was later identified as a CIA employee.
The Department of Justice launched an investigation into who had leaked her CIA connection.
Lewis "Scooter" Libby, a top aide to Vice-President Dick Cheney, was convicted of perjury and obstruction of justice in the aftermath of the leak.