President George Bush expressed no regret today in a bullish speech to mark the fifth anniversary of the Iraq invasion in which he described the war as noble, necessary and just and boasted that violence has been significantly reduced and al-Qaida's off-shoot in the country defeated.

With polls showing a clear majority of Americans opposed to the war, Bush was unrepentant and adamant US forces will remain in the country: "Five years into this battle, there is an understandable debate over whether the war was worth fighting, whether the fight is worth winning, and whether we can win it.

"The answers are clear to me: removing Saddam Hussein from power was the right decision - and this is a fight Americans must win."

The war has left tens of thousands dead, cost hundreds of millions of dollars and has been blamed for creating fresh instability in the Middle East.

Speaking to a small gathering of troops in the Pentagon, he claimed his decision in January last year to expand the number of US troops in Iraq from 124,000 to 154,000, the surge strategy, has brought a significant drop in violence and "opened the door to a major strategic victory in the broader war on terror".

Having already indicated that the number of US troops will drop to about 140,000 in the summer, he hinted he is not planning any further withdrawals when the US commander in Iraq, General David Petraeus, returns to Washington to review progress next month.

There were small protests throughout Washington today but a promised mass demonstration failed to materialise.

Bush's comments amounted to his most upbeat assessment of the Iraq war since his famous 'mission accomplished' speech on a US aircraft carrier in May 2003.

Since then, Iraq has been convulsed by Sunni and Shia Muslim insurgencies against the US-led coalition and vicious sectarian tit-for-tat killings.

The Iraq Body Count group, in a survey on behalf of the Guardian that looked at the death toll province by province, today put the number of civilian deaths at 89,322.

These are based on reported deaths, as opposed to other surveys that offer estimates and put the toll much higher, between 600,000 to 1 million.

The number of US military killed stands at 3,990, and the British toll at 175.

Bush made no mention today of the failure to find Saddam Hussein's alleged weapons of massive destruction, the stated reason for war, but returned to the theme, warning that that a hasty US withdrawal from Iraq could lead to an emboldened al-Qaida with access to Iraq's oil resources pursuing "its ambitions to acquire weapons of mass destructions to attack America and other free nations".

The cost of the war is estimated at between half a trillion dollars to three trillion. Bush, in his speech, claimed the latter figure was exaggerated, but did not provide a figure of his own.

Bush launched the war with a surprise air strike aimed at taking out Saddam Hussein but the Iraqi president had left the location. The invasion was a time of high confidence in the White House, with the hawks and neo-conservatives certain that their project of establishing Iraq as a model of democracy in the Arab world would prevail.

With less than a year left in office, Bush is a largely isolated figure. Most of the hawks and neo-conservatives who pressed him to go to war, such as the then defence secretary, Donald Rumsfeld and his deputy Paul Wolfowitz, have gone.

The fragile multi-national coalition Bush put together has also largely disappeared, with countries having either abandoned the war or left token forces behind.

In spite of continuing bombings and the criss-crossing of Baghdad with high cement walls that have left communities cut off, there is some evidence to support Bush's claim that the surge strategy is working, with the US reporting the number of attacks on its forces down from 1,580 a week in June last year to about 570 now. Sectarian violence is also reported by the US administration to be down.

Bush today attributed this not just to the deployment of more US troops but the alliance of US and Iraqi forces with local tribal and community leaders throughout the country against al-Qaida, whose Iraqi off-shoot has been responsible for some of the most gruesome atrocities.

Bush is preparing to leave the problem to his successor, who will take office in January next year.

The Republican candidate, John McCain, who visited Iraq this week and has said the US would maintain a military presence in Iraq for 100 years if need be, said today: "America and our allies stand on the precipice of winning a major victory against radical Islamic extremism."

Both the Democratic candidates, Barack Obama and Hillary Clinton, have promised to withdraw troops.

Obama has said all US combat troops would be out by the end of next year while Clinton has said troop withdrawals would begin within 60 days of her becoming president. "We cannot win their civil war. There is no military solution," she said today.

In words addressed to Obama and Clinton, Bush said: "The successes we are seeing in Iraq are undeniable - yet some in Washington still call for retreat. War critics can no longer credibly argue that we are losing in Iraq."

He warned that leaving Iraq prematurely would allow al-Qaida to grow in strength inside its borders. It would also embolden Iran in its bid to secure a nuclear weapon capability. And, in a new argument reflecting US and world concern over impending recession, it would threaten the world economy.

"If we were to allow our enemies to prevail in Iraq, the violence that is now declining would accelerate - and Iraq could descend into chaos. Al-Qaida would regain its lost sanctuaries and establish new ones - fomenting violence and terror that could spread beyond Iraq's borders, with serious consequences to the world economy," he said.