The Marshall Islands has requested U.S. assistance in securing the release of MV Maersk Tigris, State Department spokeswoman Marie Harf told reporters in Washington.

She said, as quoted by CNN, that the U.S. is communicating with officials of the islands and of the shipping company “about how best to effect that outcome.”

She said no Americans were aboard the seized ship, according to information from its officers.

Under agreements with the Marshall Islands, the U.S. “has full authority and responsibility” for security and defense matters, Harf said, “including vessels flying their flag.”

Naval forces of Iran’s Islamic Revolutionary Guard Corps ordered the ship farther into Iranian waters Tuesday as it was transiting the Strait of Hormuz. When the ship’s captain refused to comply, Iranians fired warning shots across the ship’s bridge, then boarded the ship and brought it closer to shore.