Addressing US military personnel, he said that the only way terrorists would win was if the US lost its nerve and abandoned Iraq and the Middle East.

Mr Cheney is the latest senior member of the US administration trying to bolster support for the war in Iraq.

On Thursday, President George W Bush will once again address the issue in a major speech in Washington.

The situation in Iraq remains the Bush administration's biggest challenge and all its senior figures have been brought out to defend the policy as public support for the war continues to slide.

Mr Cheney said that the threat of terrorism would be removed as people in Iraq and the wider Middle East took control of their own lives.

But he added, in a direct appeal to the American people, that like other great duties in history, it would require decades of patient effort.

'Civil war'

The vice president insisted that progress was being made in training up Iraq's own security forces, though he did not indicate how long US forces would remain.

President Bush has promised that America will stay on the offensive to prevent insurgents from disrupting next week's referendum on Iraq's new constitution.

But as the number of US military personnel killed rises towards 2,000, the grumblings are getting louder.

In a letter to President Bush, Democrat senators have warned that continuing on the same path in Iraq could lead to a full-blown civil war.