Moscow on Monday called for the release of a Russian owned, U.S. sanctioned vessel which was detained by South Korean port authorities on Saturday.

Russia’s foreign ministry announced it had summoned the South Korean ambassador and demanded Seoul release the cargo ship.

“The Russian side demanded the (South Korean) maritime authorities’ ban on the vessel leaving the port be immediately canceled,” the ministry said in a statement on its website.

The exact reasons for the ship’s detention are unclear, though the 8723-tonne general cargo ship called Sevastopol was designated by the U.S. Treasury Department’s Office of Foreign Assets Control on