South Korea announced Sunday that it has seized another ship suspected of violating U.N. sanctions by selling oil to North Korea at sea – this time a Panama-flagged one.

The announcement Sunday came just days after South Korea said it is holding the crew of a Hong Kong-flagged ship that allegedly handed over oil to a North Korean vessel in October.

The 5,100-ton ship, named the KOTI, is being held in the western port of Pyeongtaek-Dangjin, maritime officials told Yonhap News Agency on Sunday.

One official said the vessel has been blocked from departing the port since Dec. 21. The KOTI’s crew are mostly from China and Burma, other officials added, declining to elaborate further on an ongoing investigation.

The other ship being investigated, the Lighthouse Winmore, is being kept in Seoul. The ship is believed to have transferred about 600 tons of refined petroleum products to a North Korean ship in international waters in the East China Sea on Oct. 19, after leaving the South Korean port of Yeosu, a South Korean Foreign Ministry official told the Associated Press on Friday.

South Korean customs authorities said they boarded the ship and interviewed crew members after they returned to Yeosu on Nov. 24. South Korea formally seized the ship after the U.N. Security Council on Dec. 22 imposed new sanctions on North Korea that allow member states to seize, inspect and freeze vessels that are suspected of transferring banned goods to or from North Korea, the official said. He spoke to the Associated Press on condition of anonymity, citing office rules.

North Korea has come under heavy sanctions imposed by the U.N. Security Council as it accelerated efforts to expand its nuclear weapons and ballistic missiles program. In recent months, the North conducted its most powerful nuclear test to date and flight-tested intercontinental ballistic missiles three times, raising concerns it’s closer than ever toward gaining a military arsenal that can viably target the United States.

Ship-to-ship trade with North Korea at sea is prohibited under U.N. sanctions adopted on Sept. 11.