South Korea is investigating a vessel that allegedly violated United Nations sanctions on North Korea, Seoul's foreign ministry said Wednesday, after it was reportedly seized over the weekend.

The investigation comes after Moscow last week used its veto power to effectively end official UN monitoring of sanctions on North Korea amid a probe into alleged arms transfers between Moscow and Pyongyang.

South Korean authorities seized the 3,000-ton cargo ship known as the DEYI on Saturday -- which was not registered to a country -- in waters off the country's south coast, Seoul's Yonhap news agency reported.

It was held while reportedly "en route to Russia from the North via China", the agency report said, citing security sources.

"Our government is conducting an investigation, based on close cooperation with the United States, regarding the ship's alleged violation of Security Council sanctions resolutions" against North Korea, Seoul's foreign ministry said in a statement.

"As the investigation is currently ongoing, it is difficult to provide details," it added.

Thirteen people were aboard the ship, including a Chinese captain and Chinese and Indonesian crew members, Yonhap reported.

North Korea has been under mounting sanctions since 2006, put in place by the UN Security Council in response to its nuclear programme.

Since 2019, Russia and China -- the North's traditional allies -- have tried to persuade the Security Council to ease the sanctions, which have no expiration date.

Pyongyang has moved to take advantage of gridlock at the United Nations, ramping up missile tests and weapons development and declaring itself an "irreversible" nuclear power in 2022.

Russia's recent ending of the UN monitoring is a major win for North Korean leader Kim Jong Un, according to experts.