The defense ministers of Japan, the United States and South Korea agreed Friday they will jointly work to ensure U.N. sanctions against North Korea are strictly enforced, including blocking ship-to-ship transfers of goods.

Japanese Defense Minister Takeshi Iwaya told reporters that he agreed with U.S. Defense Secretary James Mattis and South Korean Defense Minister Jeong Kyeong Doo that cooperation among their defense authorities is necessary for North Korea's denuclearization.

The trilateral meeting came ahead of a possible second summit between U.S. President Donald Trump and North Korean leader Kim Jong Un, which Trump expects to realize in the near future.

The historic U.S.-North Korea summit in Singapore in June boosted expectations for denuclearization of the Korean Peninsula, but no tangible progress has been made since then.

"We'd like to have hopes but we also need to look at developments regarding North Korea's denuclearization carefully," Iwaya told his U.S. and South Korean counterparts at the meeting.

The three defense chiefs confirmed their support for diplomatic efforts in achieving North Korea's denuclearization in a complete, verifiable and irreversible manner.

Iwaya sought cooperation from South Korea, which is not currently part of international efforts to monitor North Korea's transfer of goods at sea between ships in violation of U.N. sanctions, and Jeong responded positively, according to Iwaya.

Mattis also expressed concern about China's militarization of the South China Sea during the trilateral meeting held on the sidelines of the annual ASEAN Defense Ministers-Plus meeting.

Separately, the defense chiefs of Japan and the United States held bilateral talks in which they agreed that the existing plan to relocate a U.S. military base from a residential area of Ginowan to a coastal area off Nago, both in Okinawa Prefecture, is the only viable one, the Japanese minister told reporters.

The controversial transfer of U.S. Marine Corps Air Station Futenma faces opposition in Okinawa as many local residents want the base moved out of the southwestern island prefecture.

The rift between the administration of Prime Minister Shinzo Abe and Okinawa remains as Denny Tamaki, who is against the transfer based on a 1996 bilateral agreement, was elected as new Okinawa governor in September following the death in August of his predecessor Takeshi Onaga, also a staunch opponent of the base's relocation within the prefecture.

Tamaki is seeking to continue dialogue with the central government and is calling for the burdens of national security, such as hosting the bulk of U.S. military facilities, to be shared across the country.

During Friday's meeting, Iwaya explained to Mattis recent developments over the relocation as the central government took countermeasures against Okinawa, home to the bulk of U.S. military facilities in Japan.

The government asked the land ministry on Wednesday to review and invalidate the Okinawa government's decision that has suspended landfill work for the base relocation.

The defense ministers from the 10 member states of the Association of Southeast Asian Nations have gathered with their counterparts from eight nations outside the region -- the United States, China, Japan, India, South Korea, Russia, Australia and New Zealand.