WASHINGTON (AP) — A North Korea-watching website says recent overhead images show two new North Korean frigates, the largest surface combat ships the nation’s navy has constructed in a quarter-century.

The website, 38 North, says the frigates could carry one helicopter each and appear designed to counter South Korean submarines or protect fisheries. They are equipped with anti-submarine rocket launchers.

The vessels can be seen in commercial satellite images from December and January. One is berthed at a shipyard in the west coast port of Nampo (NAM-poh), the other at a shipyard at the northeastern port of Nanjin (NAH-jin).

It’s unclear whether the frigates are yet ready for service.

The website notes that North Korea has constructed the vessels despite economic sanctions and reported industrial stagnation.