An unreleased United Nations report has stated that North Korea has been supplying materials that can be used to develop chemical weapons to Syria, The New York Times reported on Tuesday. The development follows reports of the suspected use of chlorine gas by the Bashar al-Assad-led Syrian government. Pyongyang has reportedly been supplying technical assistance to Damascus for more than a decade.

The materials supplied include acid-resistant tiles, corrosion-resistant valves and pipes. Pyongyang’s missile experts have been identified at Syrian weapon-manufacturing premises, media reports said. Syrian officials, however, claimed that the experts were coaches and athletes “confined in the area of sports.”

The report stated that five of the allegedly several shipments sent to Syria have been made via a Chinese trading firm in 2016 ad 2017, the The Wall Street Journal reported.

The Syrian government has, in the past, denied allegations that it has used chemical weapons. In a statement to the United Nations expert panel, Syria said there are “no Democratic People’s Republic of Korea technical companies in Syria”, BBC reported.

The United Nations has imposed sanctions, and restrictions on North Korea’s nuclear activity. United Nations spokesperson Stéphane Dujarric did not comment on the leaked report. “I think the overarching message is that all member states have a duty and responsibility to abide by the sanctions that are in place,” Dujarric told The New York Times.

In June 2017, a report by the international chemical weapons watchdog confirmed that the banned nerve agent Sarin gas was used in the deadly attack in Northern Syria on April 4, 2017.