South Korean authorities have seized a Panama-flagged vessel suspected of transferring oil products to North Korea in violation of international sanctions, a customs official said on Sunday.

The Russian Foreign Ministry on Saturday denied claims that U.N. sanctions against North Korea had been breached by Russian tankers transferring fuel to North Korean tankers at sea.

The sales of oil or oil products from Russia, the world's second biggest oil exporter and a veto-wielding member of the United Nations Security Council, breach U.N. sanctions, the security sources said.