Japanese Prime Minister Shinzo Abe has told US Vice President Joe Biden he would have "serious concerns" if WikiLeaks claims Washington spied on Japanese politicians were true.
A senior Japanese official said Mr Biden had apologised to the prime minister in a telephone call for "causing troubles", without confirming the spying claims.
The whistleblower group said on Friday that it had intercepted revealing years-long spying by the US National Security Agency (NSA) on Japanese officials and major companies.
Mr Abe has "also requested that the case be investigated and (Washington) supply an explanation".
Tokyo's initial response was widely seen as muted compared to the anger expressed in France and Germany following similar NSA spying allegations.
Japan is one of Washington's key allies in the Asia-Pacific region and they regularly consult on defence, economic and trade issues.
Following the call between Mr Abe and Mr Biden, the White House issued a statement highlighting the countries' strong ties.
Unlike German chancellor Angela Merkel and French president Francois Hollande, Mr Abe did not appear to be a direct target of wire tapping - but other senior politicians were, according to WikiLeaks, including Trade Minister Yoichi Miyazawa.
Bank of Japan governor Haruhiko Kuroda was also in the sights of US intelligence, WikiLeaks said.
The allegations came just as delegates negotiating a vast free-trade agreement known as the Trans-Pacific Partnership failed to reach a final deal after several days of intense talks in Hawaii.
Wikileaks' allegations also come at a time when Mr Abe has moved to expand the role of Japan's military, a move applauded by Washington but one that is deeply unpopular at home.
WikiLeaks said the US intercepts showed "intimate knowledge of internal Japanese deliberations" on trade issues, nuclear policy, and Tokyo's diplomatic relations with Washington.
"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," it said.
