Seoul (CNN) South Korea has seized a second ship as part of what it describes as an ongoing effort to monitor North Korea's attempts to evade UN sanctions.

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.

The ship, which can carry 5,100 metric tons of oil, is currently docked in Pyeongtaek-Dangjin port on South Korea's west coast after security officials reportedly decided on December 21 not to allow the vessel to leave the port, according to Yonhap citing a local maritime office.

The South Korean Foreign Ministry told CNN Sunday that authorities have launched an investigation into a ship it has seized, but it would not give further details "due to the sensitivity of the issue," about whether that vessel was suspected of transferring oil to North Korea in violation of international sanctions on the hermit kingdom.

"We are closely monitoring North Korea's attempts to evade UN sanctions. We are also working closely with relevant countries and ministries for a thorough implementation of the UN resolutions. As for the KOTI, an investigation is underway," a foreign ministry spokeswoman told CNN by telephone.

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