The United States is preparing to use the toxic riot-control agents CS gas and pepper spray in Iraq in contravention of the Chemical Weapons Convention, provoking the first split in the Anglo-US alliance.

"Calmative" gases, similar to the one that killed 120 hostages in the Moscow theatre siege last year, could also be used.

The convention bans the use of these toxic agents in battle, not least because they risk causing an escalation to full chemical warfare.

This applies even though they can be used in civil disturbances at home: both CS gas and pepper spray are available for use by British police.

The US Marine Corps confirmed last week that both had already been shipped to the Gulf.

It is British policy not to allow troops to take part in operations where riot-control agents are used.

But US Defence Secretary Donald Rumsfeld has asked President George W. Bush to authorise their use. Bush is understood to have agreed.

Internal Pentagon documents also show the US is developing a range of calmative gases - including sedatives such as the benzodiazepines and new drugs that affect the nervous system - also banned for battlefield use.

US defence sources predict these could be used in Iraq by elite special forces units to attack command and control bunkers deep underground.

Rear-Admiral Stephen Baker, senior adviser to the Centre for Defence Information in Washington, said US special forces had knock-out gases that could "neutralise" people.

"I would think that if they get a chance to use them, they will."

The Pentagon said last week that the decision to use riot-control agents "is made by the commander in the field".

Rumsfeld became the first senior figure on either side of the impending conflict to announce his wish to use chemical agents in a little-noticed comment to the House of Representatives Armed Services Committee on February 5.

He attacked the "straitjacket" imposed by bans in international treaties on using the weapons in warfare and specified that they could be used "where there are enemy troops in a cave [and] you know there are women and children in there with them".

General Richard Myers, chairman of the US Joint Chiefs of Staff, spoke of using them against human shields.

The revelations leave the Bush Administration open to charges of double standards at a time when it is making Iraq's suspected arsenal of chemical and biological weapons the casus belli.

Leading experts and Whitehall officials fear that using even pepper spray and CS gas would destroy the credibility of the Chemical Weapons Convention, provoke Iraqi chemical retaliation and set a disastrous legal precedent.

Professor Julian Perry Robinson, an authority on the convention, said: "Legally speaking, Iraq would be totally justified in releasing chemical weapons over the United Kingdom if the alliance uses them in Baghdad.

"When the war is over and these things have been used they will have been legitimised as a tool of war, and the principle of toxic weapons being banned will have gone."

The Ministry of Defence has warned the US that it will not allow British troops to be involved in operations where riot-control agents are used, or to transport them to the battlefield, but Britain is even more concerned about the calmatives.

A special working group of the Federation of American Scientists concluded last month that using even the mildest of these weapons to incapacitate people would kill 9 per cent of them. It added: "Chemical incapacitating weapons are as likely as bullets to cause death."

© NZ Herald