South Korean authorities have seized a Panama-flagged vessel suspected of transferring oil products to North Korea in violation of international sanctions, a customs official said on Sunday.

The KOTI was seized at Pyeongtaek-Dangjin port, the official told Reuters, without elaborating, due to the sensitivity of the issue. The port is on the west coast south of Incheon.

The ship can carry 5,100 tonnes of oil and has a crew mostly from China and Myanmar, Yonhap News Agency reported, adding that South Korea's intelligence and customs officials are conducting a joint probe into the vessel.

The official did not say when authorities moved in, but the seizure was the second to be revealed within just a few days.

South Korea said on Friday that in late November it had seized the Hong Kong-flagged Lighthouse Winmore, which is suspected of transferring as much as 600 tons of oil to the North Korea-flagged Sam Jong 2.

The UN Security Council last month unanimously imposed new sanctions on North Korea for a recent intercontinental ballistic missile test, seeking to limit its access to refined petroleum products and crude oil.

The United States has also proposed that the United Nations Security Council blacklist 10 ships for transporting banned items from North Korea, according to documents seen by Reuters on Tuesday.

The Lighthouse Winmore is one of the 10 ships proposed to be blacklisted. The KOTI does not seem to be included on the list.

China on Friday denied reports it had been illicitly selling oil products to North Korea in defiance of UN sanctions, after US President Donald Trump said he was unhappy that China had allowed oil to reach the isolated nation.