Officials seized the KOTI, a Panamanian-flagged vessel, after it was suspected of illegally selling oil to North Korea, maritime authorities told South Korea's Yonhap news agency.
A Panama-flagged tanker named Koti, has been seized by South Korean authorities who suspect that the ship may have supplied oil to its neighbours, North Korea in violation of global sanction.
South Korea seized the second ship, Koti, at Pyeongtaek-Dangjin, a port on its west coast.
The Russian foreign ministry has denied claims that United Nations sanctions against North Korea had been breached by Russian tankers transferring fuel to North Korean tankers at sea.
The vessel is suspected of transferring 600 tonnes of oil to the North Korean tanker Sam Jong 2 in worldwide waters on October 19.
The Lighthouse Winmore is one of 10 ships the U.S. has asked the Security Council to blacklist for violating sanctions against North Korea. The Hong Kong-registered Lighthouse Winmore, which was chartered by a Taiwanese company, was seized at the South's port of Yeosu in November for allegedly transferring refined petroleum products to the North's Sam Jong 2.
The US Treasury Department released satellite imagery in November of two ships allegedly performing an illegal ship-to-ship transfer in worldwide waters on the same day.
Earlier this month, the US requested the U.N. Security Council to deny access to ports worldwide to 10 ships suspected of violating trade sanctions on North Korea.
It did not include even harsher measures sought by Washington that would ban all oil imports and freeze worldwide assets of the government and its leader Kim Jong Un. "We are liaising with the Korean parties concerned to obtain further information about the incident, and will take appropriate actions as necessary", the statement said.
The Security Council has imposed three sets of sanctions on North Korea this year: One on Aug 5 targeting the iron, coal and fishing industries, another set on Sep 11 aimed at textiles and limiting oil supply, and the most recent on Dec 22 focused on refined petroleum products.
CNN's Taehoon Lee reported from in Seoul, Lauren Said-Moorhouse wrote from London while Yuli Yang and James Griffiths contributed from Hong Kong.