North Korea has been sending equipment to Syria that could be used in chemical weapons manufacturing, US media report, citing findings by UN experts.
The materials include acid-resistant tiles, valves and pipes, reports say.
An as-yet-unreleased UN report also says Pyongyang's missile specialists have been seen at Syrian weapon-making facilities, the New York Times says.
The allegations come after new reports of chlorine gas being used by Syrian forces, which the government denies.
North Korea is under international sanctions over its nuclear programme.
The supplies reported to have been illicitly sent to Syria by North Korea included high-heat, acid-resistant tiles, corrosion-resistant valves and thermometers. The tiles are said to be used to construct facilities where chemical weapons are produced.
Five shipments were sent to Syria via a Chinese trading firm during late 2016 and early 2017, the Wall Street Journal says. They are allegedly part of dozens of shipments over several years.
The Scientific Studies and Research Center (SSRC) - a Syrian government agency - is said to have paid North Korea via a number of front companies, the newspaper reports.
The report - also seen by the Washington Post - was compiled by the UN Panel of Experts, which assesses North Korea's compliance with UN resolutions.
In a September 2017 report, which is publicly available, the group said it was "investigating reported prohibited chemical, ballistic missile and conventional arms cooperation" between Syria and North Korea.
It said that two UN member states had intercepted shipments bound for Syria, and that the goods were suspected to be supplied by North Korea's main arms exporter as part of a contract with front companies representing the SSRC.
UN spokesman Stéphane Dujarric did not say whether the leaked report would be published, but told the New York Times: "I think the overarching message is that all member states have a duty and responsibility to abide by the sanctions that are in place."
The Syrian government is reported to have told the UN panel that the only North Koreans present in Syria are sports coaches and athletes.
Syria signed up to the Chemical Weapons Convention and agreed to have its declared chemical weapons stock destroyed in 2013 after a sarin gas attack killed hundreds of people in Ghouta, reports BBC.
It has been accused of repeatedly using banned chemical weapons in the civil war since then.