President Joe Biden announced Friday that the United States has fully destroyed its decades-old stockpiles of chemical weapons, a milestone hailed as completing the elimination around the world of all known stores of the agents of mass death.

"Today, I am proud to announce that the United States has safely destroyed the final munition in that stockpile -- bringing us one step closer to a world free from the horrors of chemical weapons," Biden said.

The United States was the last of the signatories of the Chemical Weapons Convention, which came into effect in 1997, to complete the task of destroying their "declared" stockpiles, though some states are believed to maintain secret reserves of chemical weapons.

The Organisation for the Prohibition of Chemical Weapons called the milestone a "historic success" of disarmament, more than one century after the uncontrolled use of chemical gases during World War I produced mass deaths and maiming of troops.

The US announcement meant that all the world's declared chemical weapons stockpiles were "verified as irreversibly destroyed," the OPCW said.

"I congratulate all States Parties, and the United States of America in this instance, on this major achievement for the international community," said OPCW Director-General Fernando Arias.

Biden said it was the first time "an entire category of declared weapons of mass destruction" has been verified as destroyed.

- Lethal mustard gas, sarin, VX -
The announcement came after the Blue Grass Army Depot, a US Army facility in Kentucky, recently completed its four-year job of eliminating some 500 tonnes of lethal chemical agents, the last batch held by the US military.

The US had held for decades stores of artillery projectiles and rockets that contained mustard gases, VX and sarin nerve agents, and blister agents.

Such weapons were condemned widely after their use with horrendous results in World War I.

They were not used significantly in World War II, but many countries retained and further developed them in the years afterward.

The most prominent use since the 1970s was Iraq's nerve gas attacks on Iran during their war in the 1980s.

More recently, the Syrian regime of Bashar al-Assad used chemical weapons on opponents during the country's civil war, according to the OPCW and other bodies.

- Doubly dangerous work -
The Chemical Weapons Convention, agreed in 1993 and coming into effect in 1997, gave the United States until September 30 this year to destroy all of its chemical agents and munitions.

Other signatories to the pact had already eliminated their holdings -- altogether some 72,000 tonnes since the treaty came into effect, according to the OPCW.

According to the US Arms Control Association, in 1990 the United States held nearly 28,600 tonnes of chemical weapons, the world's second largest store after Russia.

With the ebb of the Cold War the superpowers and other countries joined together to negotiate the Chemical Weapons Convention.

Eliminating the stockpiles, doubly dangerous because it means neutralizing not only the chemical agents but also the munitions they are contained in, was a slow process.

Russia completed destroying its declared stockpiles in 2017.

By April 2022, the US had less than 600 tonnes left to destroy.

Biden called for continued vigilance to ensure all chemical weapons around the world are destroyed and for the four countries that haven't signed or ratified the treaty -- Egypt, Israel, North Korea and South Sudan -- to do so.

Currently four signatory countries are considered not in compliance on suspicion of having undeclared stockpiles: Myanmar, Iran, Russia and Syria.

"Russia and Syria should return to compliance with the Chemical Weapons Convention and admit their undeclared programs, which have been used to commit brazen atrocities and attacks," Biden said.