A government panel on Monday called for integrating a forest area in northern Okinawa, including land returned by the U.S. military, with an adjacent national park to better conserve the environment.

The panel asked the Environment Ministry incorporate some 3,700 hectares of forest area partly used as a U.S. training site into Yambaru National Park at the northern end of the main island of Okinawa. The U.S. returned the land in December 2016.

The national park boasts one of the largest subtropical forests in Japan and provides habitats for species endemic to Yambaru, such as the endangered Okinawa rail and Okinawa woodpecker. Adding the new land would bring the area covered by the park to about 20,000 hectares.

The park is part of an island chain in the southwest that Japan asked the United Nations Educational, Scientific and Cultural Organization to designate as a World Natural Heritage site in February 2017.

But the former U.S. military training site was excluded from the plan due to insufficient preparation.

Earlier this month, a UNESCO panel recommended another review of the proposal, noting it should also cover the forest in the former U.S. training site.

Okinawa reverted to Japanese control in 1972 after it was occupied by the U.S. after the end of World War II. It hosts the bulk of U.S. military facilities in Japan.