The use of chemical and biological weapons was outlawed in 1925, in the Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare.

This did not, however, ban their development and stockpiling, which the US, Soviet Union and many other countries did extensively throughout the decades that followed.

Russia has said it will destroy its biological and chemical weapons

In the last 30 years most of the world's nations have signed later treaties agreeing to halt development and to destroy what arsenals they have.

But although enforcement bodies exist to verify destruction of the weapons, it is still impossible to prove that these nations are not retaining some stocks.

And fears remain about the countries that have refused to sign.

Iraq is of particular concern, as it used chemical weapons extensively in the closing stages of the 1980-88 war with Iran and against Kurds in northern Iraq.

It has also admitted manufacturing mustard gas, and the nerve agents VX, sarin and tabun, as well as the biological agent anthrax and the toxins botulinum toxin, ricin and aflatoxin.