SEOUL, Korea, Republic Of -- South Korean authorities have seized a Panama-flagged oil tanker suspected of evading international sanctions by trading with North Korea, officials said Sunday.

An official from South Korea's Foreign Ministry said the 5,100-ton KOTI and its crew members were being held at Pyeongtaek-Dangjin port on the country's west coast amid an inspection over supposed "North Korea-related" activity. The official didn't elaborate on the specifics of the activities or when and where they allegedly took place. She spoke on condition of anonymity, citing office rules.

An official from the Pyeongtaek Regional Office of Oceans and Fisheries, who also requested anonymity, confirmed that the vessel had been disallowed from leaving the port since Dec. 21.

The revelation comes days after South Korea said it was holding a Hong Kong-flagged ship and crew members over transferring petroleum products to a North Korean vessel in international waters in October. The Lighthouse Winmore is believed to have transferred about 600 tons of refined petroleum products to the North Korean ship in international waters in the East China Sea, according to the Foreign Ministry.

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

North Korea has come under heavy sanctions imposed by the UN Security Council as it has accelerated efforts to expand its nuclear weapons and ballistic missile program.

In recent months, North Korea conducted its most powerful nuclear test to date and flight-tested intercontinental ballistic missiles three times, raising concerns that it's closer than ever to gaining a military arsenal that can viably target the mainland United States.