TOKYO -- The Japanese government is working with Asian neighbors as well as faraway countries like Canada and the U.K. to clamp down on North Korean smuggling to ensure that United Nations sanctions continue to pressure the Pyongyang regime.

To gain access to sanctioned goods like petroleum, North Korean vessels have been transferring cargo to and from other countries' ships on the high seas. Australia and Canada have recently joined Japan and the U.S. in patrols against these operations, flying surveillance planes out of the Kadena Air Base in Okinawa. The British also deployed naval frigate HMS Sutherland to the waters near Japan in early May.

Japan hosted the Pacific Islands Leaders Meeting on Friday and Saturday. The 19 participating countries and territories adopted a declaration expressing "deep concerns about North Korea's sanctions evading tactics, including 'ship-to-ship' transfers," though some members of the framework, like Samoa, the Marshall Islands and Palau, are suspected of involvement themselves.

Japan's Self-Defense Forces and Coast Guard are conducting their own patrols, mainly in the East China Sea. They have published photographs from four incidents this year where they found a North Korean tanker alongside a vessel registered in another country. The last photographs were published in Feb. 24. Tokyo remains vigilant, concerned that Pyongyang is inventing even more elaborate schemes to circumvent sanctions.

The heightened surveillance is intended to warn Kim Jong Un ahead of his planned June 12 summit with U.S. President Donald Trump that sanctions will continue until he takes concrete steps toward denuclearization. "Ship-to-ship transfers must not become a loophole, and Japan will take a leading role" to prevent this, Prime Minister Shinzo Abe said.

The Japanese government worries that international pressures against the North could ease following the recent inter-Korean summit. The Maritime SDF found a South Korean tanker and North Korean ship next to each other in the East China Sea in early May. The South Korean government denied any involvement in ship-to-ship transfers, but an MSDF source still described the incident as "suspicious."

"We have to maintain pressure against North Korea through the economic sanctions based on the U.N. Security Council resolutions, and must not make a mistake on the timing of easing sanctions," Japanese Foreign Minister Taro Kono said Sunday.