The U.S. Navy invited South Korean and Japanese admirals aboard an Ohio-class nuclear submarine last month, according to new Pentagon photos released on Wednesday, in what one expert framed as part of U.S. efforts to encourage trilateral cooperation against North Korea and other threats.

Rear Admiral Su Youl-lee, commander of ROK submarine forces, and Japanese Vice Admiral Tateki Tawara were among the officials from the two countries who rode the USS Maine near Guam and reviewed its ballistic missile capabilities on April 17, according to the Pentagon.