Japanese Prime Minister Shinzo Abe has told US Vice President Joe Biden he will have "serious concerns" if WikiLeaks claims Washington spied on Japanese politicians are true, calling for an investigation.
Tokyo's Cabinet spokesman Yoshihide Suga said Biden had apologised to the Japanese leader in a telephone call for "causing troubles", without confirming the spying claims.
WikiLeaks said on Friday (local time) it had intercepts revealing years-long spying by the US National Security Agency (NSA) on Japanese officials and major companies.
"Prime Minister Abe told [Biden] that, if figures in Japan were in fact subject to these activities, it would risk jeopardising the trusting relations between allies and he would have to express serious concern," Suga told a press briefing.
Abe "also requested that the case be investigated and [Washington] supply an explanation".
On Monday, Suga called the claims "deeply regrettable", but Tokyo's 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 Abe-Biden call, the White House issued a statement highlighting the countries' strong ties.
"In the call, the vice president underscored our strong commitment to the US-Japan alliance and thanked Prime Minister Abe for his enduring partnership," it said.
"The vice president reaffirmed the United States' commitment made by President Obama in a 2014 presidential directive to focus our intelligence collection on national security interests."
Unlike German chancellor Angela Merkel and French president Francois Hollande, Abe did not appear to be a direct target of wiretapping 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 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.