The US spied on Japanese politicians, its top central banker and major firms including conglomerate Mitsubishi, according to documents released by WikiLeaks on Friday, in the latest revelation about Washington's snooping on allies.

The intercepts exposing US National Security Agency (NSA) espionage follow other documents that showed spying on allies including Germany and France, straining relations with them.

Japan is one of Washington's key allies in the Asia-­Pacific region and they regularly consult on defense, economic and trade issues.

"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.

"The documents demonstrate intimate knowledge of internal Japanese deliberations" on trade issues, nuclear and climate change policy, and Tokyo's diplomatic relations with Washington, it said.

The group also pointed to intercepts about a "sensitive climate change strategy" and the "content of a confidential prime ministerial briefing that took place at [Prime Minister] Shinzo Abe's official residence."­

There is no specific mention of wiretapping Abe but senior politicians were targeted, including Trade Minister Yoichi Miyazawa, while Bank of Japan governor Haruhiko Kuroda was also in the sights of US intelligence, WikiLeaks said.

The claims of spying on trade officials come at a particularly sensitive time after high-profile talks kicked off recently in Hawaii aimed at hammering out a vast free-trade bloc encompassing 40 percent of the world's economy.