President Joe Biden announced that the Blue Grass Army Depot, a US Army facility in Kentucky, had eliminated its decades-old stocks, completing a global effort started in 1997 to rid the world of chemical weapons.

"The end of destruction of all declared chemical weapons stockpiles is an important milestone", Organisation for the Prohibition of Chemical Weapons (OPCW) chief Fernando Arias said in a statement.

The Hague-based body said that the step by the United States, the "last possessor state", meant that "all declared chemical weapons stockpiles (were) verified as irreversibly destroyed".

But recent use of chemical weapons meant the world still had to be on guard, the Nobel Peace Prize-winning OPCW warned.

The watchdog has in recent years blamed Syria for carrying out chemical attacks during its civil war, and investigated the use of Soviet-era nerve agents against a former Russian spy in Britain and Kremlin opponent Alexei Navalny in Russia.

"Recent uses and threats of use of toxic chemicals as weapons illustrate that preventing re-emergence will remain a priority for the organisation," Arias cautioned.