MOSCOW (Reuters) - Russia's Ministry of Foreign Affairs summoned South Korea's ambassador in Moscow on Monday and demanded that Seoul allow a Russian vessel to leave South Korea's port of Busan, the ministry said.

The 'Sevastopol' has been illegally held at the port, the ministry said without adding what had served as the pretext for the alleged detention.

"The Russian side demanded the (South Korean) maritime authorities' ban on the vessel leaving the port be immediately canceled," it said in a statement.

A vessel named Sevastopol was one of six Russian-flagged vessels targeted by United States sanctions in August for their alleged breach of United Nations restrictions on North Korea.

Washington accused the vessels and two Russian shipping companies of involvement in the transfer of refined petroleum products to North Korean vessels.

Russia denies the allegations and has called the sanctions groundless.

(Reporting by Maria Tsvetkova; Writing by Tom Balmforth; Editing by Toby Chopra)