Defense Minister Tomomi Inada and Gen. Robert Neller, commandant of the U.S. Marine Corps, agreed Wednesday to cooperate over the return of roughly half of a large U.S. military training area in Okinawa Prefecture, Defense Ministry officials said.

The Japanese government seeks the return of the land within the United States' Northern Training Area by year-end to demonstrate it is taking steps to ease Okinawa's base-hosting burden.

"I call for continued cooperation (by the United States) as we are making diligent efforts," said Inada in Tokyo at her first meeting with Neller, according to the officials. She took office in August.

The United States agreed in 1996 to return to Japan about 4,000 of some 7,800 hectares that make up the training range, provided that six helipads will be relocated to another area.

But the outlook for the relocation plan remains uncertain as local residents oppose it due to safety concerns posed by the use of the helipads by Osprey tilt-rotor transport aircraft, which many in Okinawa view as accident-prone.

Inada also reiterated Japan's request that the United States conduct exercises involving Osprey aircraft outside Okinawa, the officials said. The aircraft are deployed at Air Station Futenma, which is located in a crowded residential area in Ginowan.

Inada and Neller also agreed to coordinate closely to deal with North Korea's development of nuclear weapons and missiles, the officials said.

The reclusive state conducted its fifth test of a nuclear explosive device last month and has repeatedly tested ballistic missiles in the face of global opposition.

"The security environment surrounding our country is extremely severe. A strong Japan-U.S. alliance is indispensable," Inada was quoted as telling Neller, who promised full support for the alliance.