The government's top spokesman on Tuesday declined to comment on — but did not deny — a report alleging that Tokyo has secretly and closely cooperated with the U.S. National Security Agency in intelligence-gathering, having been provided with an extremely powerful mass-surveillance tool that can hack and monitor emails and other private communications.

The joint report by The Intercept, an online investigative reporting organization, and NHK is based on top-secret documents provided by whistleblower and former NSA contractor Edward Snowden.

The Intercept posted on its website an April 2013 document suggesting the NSA had provided Japan with the XKeyscore tool that, according to earlier Snowden documents, gives users the ability to suck up huge amounts of online data and search through numerous private communications, including emails, documents, usernames and passwords.

The existence of XKeyscore was first reported by The Guardian in July 2013, based on documents provided by Snowden.

The NSA boasted that XKeyscore — its "widest-reaching" tool — covers "nearly everything a typical user does on the internet," The Guardian reported.

Asked about the joint Intercept-NHK report, Chief Cabinet Secretary Yoshihide Suga said the government would not comment on allegations based on documents of unknown origin. "As you know, Japan and the United States are alliance countries," he said. "We exchange various information, but we won't answer a question about specific contents given the relationship with the other country."

According to the article, Japan began sharing information with the NSA that could be used to "identify particular kinds of malicious software being used by hackers" in September 2012.

The NSA found the data "highly valuable" in protecting critical U.S. corporate information systems, and in return provided Japan with Xkeyscore while also training Japanese spies, according to the report.

The full report can be read at The Intercept's website (