North Korea is suspected of having tried to send missile components to Syria in violation of UN sanctions, but the shipment was intercepted by South Korean authorities, diplomats say.
South Korean officials seized 445 graphite cylinders, which had been declared as lead piping, from a Chinese vessel, the Xin Yan Tai, UN Security Council diplomats said. The incident occurred in May.
South Korean authorities stopped the ship at Busan, the envoys said, adding that the cylinders were intended for a Syrian company called Electric Parts.
South Korean officials briefed the Security Council's North Korea sanctions committee about the seizure on October 24, the envoys said, and China had offered to help investigate the circumstances surrounding the incident.
"It appears the cylinders were intended for Syria's missile programme," a diplomat said. "China assured us they will investigate what looks like a violation of UN sanctions."
Another diplomat said: "It's possible that the crew of the Chinese ship had no idea what this shipment really was. It's good that China has expressed a willingness to investigate."
Foreign Ministry spokesman Hong Lei said that China strictly followed UN resolutions and its own non-proliferation export controls.
"China will handle behaviour that violates relevant UN Security Council resolutions and China's laws and regulations according to the law," he said.
Diplomats said the graphite cylinders appeared to be consistent with material which could be used in a ballistic missile programme and that South Korea would be jointly investigating the case with China.
The shipment to Syria was arranged by a North Korean trading company, diplomats said. One diplomat said the Syrian company that was to have received the cylinders may be a subsidiary of the North Korean trading firm.
North Korea is barred from trading nuclear and missile technology under UN Security Council sanctions imposed on Pyongyang because of its nuclear tests in 2006 and 2009.
There are United States and European Union sanctions on Syria, but no UN arms embargo against the government of Syrian president Bashar al-Assad, who has led a 20-month military campaign against an increasingly militarised opposition.
Russia and Iran have been Assad's main arms suppliers.
Earlier this year, the Security Council's panel of experts on North Korea, an independent group that monitors compliance with the UN sanctions, said it was investigating possible weapons-related deals between Pyongyang, Syria and Myanmar.
"The DPRK [North Korea] continues actively to defy the measures in the [UN sanctions] resolutions," the panel said in May.
The cylinders could be used for rocket nozzles and re-entry vehicle nose tips, Japan's Kyodo news agency said.
The ship, registered in Shanghai, was built in 2005 and owned by a Shanghai company, Kyodo said, quoting the China Classification Society of vessels.
Reuters, Agence France-Presse