The South Korean vessel was impounded along with three other ships, AFP reported on Wednesday.
“This is the first time a South Korean vessel has been detained for allegedly violating UN sanctions,” a South Korean Foreign Ministry official told the news agency.
The ship — the 5,160-ton P-PIONEER — has been banned from leaving the southern port of Busan since last October because it is suspected of carrying out an illicit ship-to-ship transfer of oil to a North Korean tanker, according to the South’s Yonhap News Agency.
It is unclear why the seizure of the ship was not publicized at the time.
The ship would be blacklisted by the UN if it is confirmed to have made the transfer.
Ships confirmed to be “smuggling” basic goods to sanctions-hit North Korea will be denied access to the ports of any UN member country.
The UN Security Council has imposed a series of economic sanctions against Pyongyang over its nuclear and missile programs.
The US, which has been the main sponsor of the international sanctions, formed a coalition of seven countries last year to expand the surveillance of ships allegedly transferring fuel to North Korea.
Pyongyang has been involved in negotiations with Washington but that diplomacy has almost ground to a halt as the US refuses to offer any sanctions relief in exchange for the multiple measures already taken by North Korea to slow down its nuclear and missile programs.