U.S. President George W. Bush has defended the Iraq war as a "right decision" despite a high cost, and he will not order troop withdrawals beyond those already planned as thousands of Americans staged anti-war demonstrations across the nation to mark the war's fifth anniversary.

However, Bush admits that the Iraq war launched on March 20, 2003 has become "longer, harder and more costly than anticipated."

The president's address will be the basis for Bush's first troop-level decision in seven months.

He reiterates his opposition to fast withdrawal of the current 155,000 U.S. troops from Iraq.

"We have learned through hard experience what happens when we pull our forces back too fast. The terrorists and extremists step in; they fill vacuums, establish safe havens and use them to spread chaos and carnage."

A poll just released by CNN has showed that about 70 percent adults surveyed oppose the Iraq war and 60 percent Americans say that troop withdrawal should begin in months after the inauguration of the new president next year.