Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons (OPCW), said in a statement on Thursday that 90 percent of "declared chemical weapons" across the globe had been destroyed.

The OCPW chief confirmed that the destroyed chemical weapons had been verified by the chemical weapons watchdog.

"This is a major milestone that shows we are well on the way to ridding the world of chemical weapons," Uzumcu said.

The US and Russia, which have the "biggest arsenals," are set to have their stockpiles eradicated by 2020 and 2023, respectively.

"This includes stocks of unitary weapons, such as sulfur mustard, and primary precursor chemicals for producing deadly nerve agents, like sarin. All destruction activities have been completed in a safe manner," an OPCW statement released on Thursday said.

Not the end yet

The statement comes after samples obtained in Syria by OPCW inspectors earlier in May tested positive for substances needed to create toxic agents for use in chemical weapons.

In 2013, the UN Security Council ordered the destruction of all chemical weapons in Syria after allegations against both sides

In March, the UK-based Syrian Observatory for Human Rights claimed that six people, including three children, were killed due to a chlorine gas attack. Bashar al-Assad's regime has repeatedly been accused of using chlorine gas against civilians and rebel groups, as have Syrian opposition forces, prompting UN chief Ban Ki-moon to seek OPCW assistance to investigate the accusations.

Following investigations, the group reported "with a high degree of confidence" that chlorine gas had been used by Assad's regime.

Uzumcu added that "more" needed to be done in order for the OPCW to complete its mission.

"The Director-General noted that there was more work ahead, and new challenges to address, if the international community is to succeed in preventing chemical weapons from re-emerging," the OPCW statement said.