Russia and the Assad regime came under pressure to acknowledge using chemical weapons, as the global chemical weapons watchdog convened in The Hague on Monday.

Damascus has still failed to declare its chemical weapons and admit inspectors, Organisation for the Prohibition of Chemical Weapons (OPCW) chief Fernando Arias said.

The nerve agent poisoning of jailed Kremlin critic Alexei Navalny in Russia meanwhile continues to pose a "serious threat" to world efforts to eradicate chemical armaments, Arias added.

Syria denies the use of chemical weapons and insists it has handed over its weapons stockpiles under a 2013 agreement with the United States and Russia, prompted by a suspected sarin gas attack that killed 1,400 in the Damascus suburb of Ghouta.

But Syria was stripped of its OPCW voting rights in April after a probe blamed it for further poison gas attacks, and they will remain suspended until it has fully declared its chemical weapons and weapons-making facilities.

"To date Syria has not completed any of these measures," Arias told the meeting, adding that its declarations "still cannot be considered accurate and complete."

Damascus was also continuing to deny a visa to an OPCW weapons inspector, leading the organization to refuse to deploy a team there, said Arias.

He said he was arranging a meeting with Syria's foreign minister to discuss the breaches.

Russia meanwhile has been accused of failing to answer questions about the 2020 Novichok poisoning of Navalny, which Western powers have blamed on the Kremlin.

"The use of chemical weapons on the territory of the Russian Federation also poses a serious threat to the convention," Arias said.

Moscow had asked OPCW inspectors to come to Russia to investigate but Arias said the visit had not taken place due to conditions set by the Russian authorities that were stricter than those imposed by other countries.

London and Washington meanwhile pushed Moscow and Damascus on chemical weapons.

"We call again on Russia and the Assad regime to comply with their obligations," Bonnie Jenkins, the U.S. Under Secretary of State for Arms Control and International Security, said in a statement to the meeting.

British junior defense minister Annabel Goldie said Russia must not only answer questions on Navalny but also the Novichok poisoning of former KGB agent Sergei Skripal in Salisbury in 2018.

"There is no plausible explanation for these poisonings other than Russian involvement and responsibility," Goldie said.

Moscow has always denied involvement in both incidents.