The intercepts exposing US National Security Agency activities follow other documents released by the whistleblower group that revealed spying on allies including Germany and France, straining relations.
Japan is one of Washington’s key allies in the Asia-Pacific region and they regularly consult on defence, economic and trade issues.
The leaks comes as Prime Minister Shinzo Abe moves to expand the role of Japan’s military, a move applauded by Washington but deeply unpopular at home.
The claims of spying on trade officials could prove particularly sensitive after high-profile talks kicked off this week in Hawaii aimed at hammering out a vast free-trade bloc encompassing 40 percent of the world’s economy.
The United States, Japan, and 10 other Pacific Rim countries are looking to finalise the most ambitious trade deal in decades.
But Washington and Tokyo — the biggest economies in the negotiations — have sparred over auto sector access and Tokyo’s concerns about including agricultural products in the proposed Trans-Pacific Partnership.
“The reports demonstrate the depth of US surveillance of the Japanese government, indicating that intelligence was gathered and processed from numerous Japanese government ministries and offices,” WikiLeaks said.
There was “intimate knowledge of internal Japanese deliberations” on trade issues, nuclear policy, and Tokyo’s diplomatic relations with Washington, it said.
Prime Minister Shinzo Abe did not appear to be a direct target of wiretapping but senior politicians were, including Trade Minister Yoichi Miyazawa, while Bank of Japan governor Haruhiko Kuroda was also in the sights of US intelligence, WikiLeaks said.
Tokyo did not immediately react to the leaked documents.
The spying goes back at least as far as Abe’s brief first term, which began in 2006, WikiLeaks said. Abe swept to power again in late 2012.
“If this is true, Japan is going to be asking for an explanation from the US side, but it’s unlikely to have a major impact on the core of Japan-US relations,” said Yoshinobu Yamamoto, a professor of international politics at the University of Niigata Prefecture.
The leaks may provoke a strong reaction from a public wary of Abe’s bid to expand the military’s role and push through a trade deal strongly opposed by Japan’s politically powerful farm lobby.
“I think some interest groups and opposition parties will use this news to stick a spoke in the government’s wheel,” said Celine Pajon, a Japan specialist at the French Institute of International Relations.
Japanese lawmakers are debating the controversial bills that would expand the role of the military and could possibly see troops fighting abroad in defence of allies — chiefly the United States — for the first time since the end of World War II.
Abe, stressing the Japan-US alliance as the cornerstone of its diplomacy, wants to enact the bills soon, but opponents say they will drag officially pacifist Japan into foreign wars.
WikiLeaks said there were also intercepts about “sensitive climate change strategy” and the “content of a confidential prime ministerial briefing that took place at Shinzo Abe’s official residence”.
One intercepted communication suggested Japanese agriculture ministry officials were alarmed at a possible backlash from Washington over a delay in importing US cherries.
Trading giant Mitsubishi’s natural gas division and Mitsui’s petroleum unit were targeted, while four reports were classified as “Top Secret”.
One report was marked to indicate it could be released to allies Britain, Australia, Canada, and New Zealand.
“In these documents we see the Japanese government worrying in private about how much or how little to tell the United States, in order to prevent undermining of its climate change proposal or its diplomatic relationship,” WikiLeaks editor-in-chief Julian Assange said in a statement.
“And yet we now know that the United States heard everything and read everything, and was passing around the deliberations of Japanese leadership to Australia, Canada, New Zealand and the UK.”