World leaders on Wednesday urged all nations to help "banish forever the evil of chemical weapons," as the global watchdog warned decades of progress towards eliminating them was under threat.
UN chief Antonio Guterres led praise at a solemn ceremony in The Hague for the work of the Organisation for the Prohibition of Chemical Weapons (OPCW) to mark its 20th anniversary.
So far some 95 percent of the world's declared stockpiles of such arms have been destroyed by the OPCW.
And its dangerous, painstaking work to implement the April 1997 Chemical Weapons Convention won it the Nobel Peace Prize in 2013.
But amid reports of sarin, mustard and chlorine gas attacks unleashed during the bitter civil war in Syria, Guterres warned that despite "two decades of success... progress is under threat."
"In the Middle East, belligerents are breaking the norm against chemical weapons. The recent attack in Syria was a horrific reminder of this threat. There can be no impunity for these crimes," he said in a video message to the ceremony.
"For 20 years we have been allies in this cause. Now let us resolve to consign these diabolic weapons to the pages of history."
The ceremony was also attended by Dutch King Willem-Alexander and Sweden's Princess Victoria, as well as ambassadors from OPCW-member nations.
Despite the celebrations, "we cannot ignore the black cloud hanging over us", said Dutch Foreign Minister Bert Koenders, referring especially to the April 4 suspected sarin gas attack in the Syrian rebel-held town of Khan Sheikhun, the latest in several attacks.
The continued use of such weapons underscored that the OPCW as an independent, neutral body was needed "more than ever before," Koenders added.
In an unprecedented step in November, the OPCW's executive council condemned Syria's use of toxic weapons -- its first public condemnation of any of the 192 members of the convention.
- Syria the 'greatest test' -
Syria joined the OPCW in 2013 after denying for years that it had toxic arms. And while 100 percent of its declared stockpile has been destroyed in an operation involving some 30 countries, there are growing fears that the Syrian regime did not reveal the true extent of its armoury.
OPCW director general Ahmet Uzumcu acknowledged Wednesday that "our collective journey to banish forever the evil of chemical weapons has reached momentous landmarks."
But he warned that "in Syria the OPCW has faced its greatest test of commitment as well as resilience."
"Our work in Syria is not yet finished. It is of grave concern that we continue to see reports of the use of chemical weapons."
He urged Egypt, Israel, North Korea and South Sudan -- the last four countries which have not yet signed the convention -- to join up "without delay".
And with some victims in the audience, he paid tribute to those who have suffered most from such deadly weapons, first used on the battlefields of Belgium's Ypres in World War I.
The victims "remind us of the human toll when morality is recklessly abandoned and universal norms callously breached," Uzumcu said.
"The work of the OPCW represents the most effective response to such cruelty; a ray of hope illuminating a dark shadow on our history."