One of Japan’s largest naval vessels took part in sea trials this week, brandishing modifications that bring it closer to embarking fifth-generation, U.S.-made fighter jets, local media reported.

The JS Kaga, a flat deck carrier for helicopters, kicked off the trials Monday after departing Kure Naval Base in Hiroshima prefecture, the Asahi Shimbun reported Tuesday. The Kaga’s bow and flight deck were modified to accommodate F-35B Lightning II fighter jets.

The F-35B variant, employed by the U.S. Marine Corps aboard U.S. Navy amphibious assault ships, is capable of short takeoffs and vertical landings, making them suitable for Japan’s helicopter carriers, provided alterations are made.

A Japan Maritime Self-Defense Force spokesman, reached by phone Wednesday, confirmed that the Kaga was at sea Monday but would not provide additional details, citing concerns for operational security.

Modifications visible Monday on the carrier included new markings to assist with takeoffs and landings and a square bow intended to reduce turbulence and extend the flight deck’s length, the Asahi reported.

The Kaga’s flight deck has been strengthened to withstand the heat of an F-35B landing, the Self-Defense Force said. The modifications began in 2022 and may be completed by March.

The Kaga’s activities on Monday did not include F-35s, and flight operations are not expected any time soon because a second phase of modifications has yet to be scheduled, the spokesman added.

Japan, however, has been preparing to adapt the Kaga and its sister ship, the JS Izumo, to embark the fighters for years. Japan’s Ministry of Defense said the ability to “possess flexible operations” and “acquire air superiority utilizing highly capable fighters” was essential for Japan’s defense, according to a 2019 white paper.

It added that improvements to the Izumo and Kaga that allow aircraft like the F-35B to operate from their flight decks was “vital” and “the bare minimum” for the Self-Defense Forces.

In 2020, the Defense Ministry asked for about $795 million to purchase six F-35Bs from the United States and around $29 million to repair and upgrade the Izumo. The following year, the ministry requested $474 million for four more F-35Bs and $61 million to refurbish the Kaga and Izumo’s flight decks.

Additional modifications to the Izumo, including bow modifications, are scheduled for next year, Asahi reported.

In October 2021, two U.S. Marine Corps F-35Bs took off and landed aboard the Izumo, which underwent similar upgrades as the Kaga, becoming the first fighter jets to operate from a Japanese carrier since World War II.

Japan has since purchased 42 F-35Bs and expects deliveries to begin within the next few years.