The United States remains opposed to setting an "arbitrary" date for withdrawing troops from Iraq, the White House said on Wednesday after Iraqi officials called for a timetable as part of a security agreement being negotiated with Washington.

"We have always been opposed and remain so to an arbitrary withdrawal date," White House spokeswoman Dana Perino said to reporters traveling with President George W. Bush in Japan.

The United States believes those decisions should be "based on conditions on the ground" and Iraqi officials agree with that, she said.

Iraq's national security adviser on Tuesday said Iraq would not accept any security agreement with the United States unless it included dates for the withdrawal of foreign forces. But the government's spokesman said any timetable would depend on security conditions on the ground.

Their differences underscored the debate in Baghdad over the security pact with Washington that will provide a legal basis for U.S. troops to remain when a U.N. mandate expires at the end of the year

The White House said the statements from Iraqi officials about a timetable for troop withdrawal partly reflected improvements in the security situation in Iraq.

"I think that is a reflection of first and foremost the positive developments that we've seen recently in Iraq, but in addition to that, the negotiations are intensifying," Perino said.

"This is about their future and they want to take on more of their own responsibility, and we want that too," she said.

Perino said she would not put a timetable on when the security agreement might be completed.

"We want to be able to try to work this out quickly and the main reason that we want this is because our troops are going to be there past the end of this year, that's a fact," she said.