Washington has said it is prepared to use nuclear weapons if necessary to respond to any attack with weapons of mass destruction (WMD) against itself or its allies.

"The United States will continue to make clear that it reserves the right to respond with overwhelming force - including through resort to all our options," a White House strategy document released on Tuesday said.

The new document has been interpreted as a warning to Iraq

It is believed to be the first update on America's WMD policy since 1993.

BBC Washington correspondent Ian Pannell says that while the new document only restates existing policy, it has been widely interpreted as a direct warning to Iraq.

US officials said only that the passage was meant to put emphasis on the role of deterrence against such an attack.

'Essential part of defence'

Copies of the six-page strategy document were released to the media ahead of its official unveiling.

Called the National Strategy to Combat Weapons of Mass Destruction, the document is to be delivered to Congress on Wednesday.

It says the threat of overwhelming force is an essential part of defence.

It also includes a commitment to boost programmes aimed at containing the damage from any chemical, biological, radiological or nuclear attack.

The document also says that some states support terrorists and already have weapons of mass murder; it contends that they seek even more "as tools of coercion and intimidation".

"For them, these are not weapons of last resort, but militarily useful weapons of choice intended to overcome our nation's advantages in conventional forces and to deter us from responding to aggression against our friends."

Iraqi denial

In 1991, President George Bush Senior warned Baghdad that it would face the severest consequences if it attacked US forces with chemical or biological weapons.

Iraq has used chemical weapons against its own Kurdish population and is believed to have also used them in its war with Iran.

President George W Bush has repeatedly vowed to strip Baghdad of any weapons of mass destruction by force if Iraqi leader Saddam Hussein does not bow to a UN disarmament ultimatum.

UN inspectors are currently in Iraq to seek out any illegal arms held by the regime.

Iraq denies possessing banned weapons but has been accused by America of lying.