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 seizure was the second to be revealed by South Korea within a few days, as the United Nations steps up efforts to squeeze essential oil supplies to the reclusive North following its nuclear or ballistic missile tests.

The ship, 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.

A marine official also confirmed the seizure, which he said was done "recently".

The KOTI's estimated time of arrival at the port was Dec. 19, according to VesselFinder Ltd., a tracking service provider, 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.