US President George Bush has asked for an extra $25bn for military costs in Iraq and Afghanistan, reversing a previous spending pledge.
The US military is facing renewed resistance in both Iraq and Afghanistan
In a statement, the White House cited "recent developments on the ground and increased demands on our troops" as the reasons for the request.
There have been recent upsurges of violent resistance to the military occupations of both countries.
Earlier, the US said it had dropped plans to reduce troop numbers in Iraq.
It had said it would cut forces to 115,000 this month, but it now says it plans to keep 135,000 troops in Iraq until 2006.
'Money for our troops'
Mr Bush had vowed not to seek more money for Iraq before the 2 November presidential election.
The White House is now asking for an extra $25bn from Congress as a "contingency reserve fund" out of the proposed budget for the fiscal year 2005, beginning on 1 October.
That will come on top of the $160bn already allocated to Iraq and Afghanistan in President Bush's previous spending bills.
White House budget director Joshua Bolten went to Congress to brief top Republican lawmakers on the additional request.
"This is not money for Iraq, this is money for our troops, this is supporting our troops. Nobody is going to have any problem with that," House Majority Leader Tom DeLay, a Texas Republican, reportedly told journalists afterwards.
But the BBC's Pentagon correspondent Nick Childs says that previously, amid much controversy, administration officials had said that they would not be asking for more funds this year because it was impossible to work out what future needs would be.
He says Wednesday's statement - which hints that the White House will be asking for more money later - is an about-face likely to provoke a further row.
COST OF THE WAR
US military operations so far: $168bn *
Military operations (projected): $150bn-300bn
Reconstruction so far: $33bn (US $18.7bn) *
Reconstruction (projected): $50bn-100bn
Extra security: $40bn-80bn
Sources: CBO, CSIS, World Bank
* allocated for US fiscal years 2003-05
"What it demonstrates is that they tried simply to avoid showing any of the costs before the election," charged Representative David Obey, top Democrat on the House Appropriations Committee.
"Now they are asking for the least they can possibly ask for... concealing the full costs."
President Bush said in his statement that his administration would submit a more
precise spending request, known as a supplemental, "when we can better estimate precise costs".
Troops to stay
Earlier, Defence Secretary Donald Rumsfeld announced that a planned cut in troops serving in Iraq was being shelved.
"This is a difficult period, but our folks are there and are going to stay there," Mr Rumsfeld said at a Pentagon briefing on Tuesday.
He said US forces could expect to face an upsurge of violence in the run-up to 30 June, when the US will transfer sovereignty to Iraqis.
The announcement follows the bloodiest month yet for US troops in Iraq, with 129 combat deaths - more than the total number who died during the invasion in March and April 2003 itself.
The Pentagon also ordered 10,000 combat troops and 37,000 reservists and national guardsmen to replace units serving extended tours of duty.