The 'war on terror' is costing Americans approximately $7 billion US every month. According to a new Congressional report, the bill could exceed half a trillion dollars by 2010.

The Washington-based Congressional Research Service says $5.9 billion of that monthly outlay is being spent on military operations in Iraq.

That's a billion-dollar, 19 per cent increase from the Pentagon's last estimate.

According to the report by one of Congress' investigative arms, the estimated total of $570 billion US by 2010 is a conservative estimate that "assumes a gradual drawdown in U.S. troops in Iraq and Afghanistan."

Even so, the CRS report said the Pentagon will be pressed to either increase the overall size of its Army, shift people from other missions, or further increase the pace of the troop rotations if it wants to remain effective into the next decade.

Funding will still be needed to continue training Iraqi and Afghan security forces and to replace lost equipment, the research service added.

In another study, the Congressional Budget Office concluded that combat missions cannot continue at their current pace without pouring $140 billion into the training of more than 150,000 new active soldiers and reserves.

And even in that case, the process will take a decade.

The pair of reports underscoring the challenge of sustaining the war on terror come the same week U.S. President George Bush renewed his commitment to the ongoing mission.

"We were not in Iraq on Sept. 11, 2001, and Al Qaeda attacked us anyway," Bush said in a sweeping defence of his war effort this week.

"The hatred of the radicals existed before Iraq was an issue, and it will exist after Iraq is no longer an excuse."

Addressing a receptive audience at the National Endowment for Democracy in Washington, Bush said he would fight as long as necessary to defeat "the enemies of humanity."

"We will never back down, never give in, and never accept anything less than complete victory."

Since Sept. 11, 2001, the Bush administration has allocated about $361 billion for military operations, reconstruction and other programs in Iraq and Afghanistan.

The Pentagon's official count pegs the total force in Iraq at 149,000, although Brig.-Gen. Carter Ham disclosed this week that actual count is now closer to 152,000.

According to the Congressional Budget Office, the actual figure could be as high as 180,000 once troops supporting the war from other locations are taken into account.