The Defense Ministry was in a stir Tuesday over the U.S. Navy's public release of specific details on a joint missile defense exercise last week that the ministry didn't know or that it wanted to be kept secret, including the amount of time it took to inform the prime minister that an enemy state had launched a ballistic missile.

It took only one minute for the U.S. Navy and the Maritime Self-Defense Force to transmit the warning to Prime Minister Shinzo Abe's office during Friday's exercise, which involved a simulated launch of a ballistic missile aimed at Japan, according to the news release issued Monday by Mass Communication Specialist 2nd Class Matthew Schwarz, U.S. 7th Fleet Public Affairs.

Defense officials first said they could not comment on details in the statement. But after repeated questions from reporters, the ministry said it did not track how long it took to deliver the message to the prime minister and the U.S. Navy should have not known the time either.

"The main purpose of the exercise was to test if communications channels and devices work. We didn't measure the time," said a senior Defense Ministry official. "We wonder how the U.S. measured it."

"This time, we actually passed the tracking information all the way to Prime Minister of Japan Shinzo Abe's office," Schwarz said in the release.

Defense experts and politicians are focused on how quickly Japan and the U.S. can detect when a ballistic missile has been launched as the two countries develop a joint missile defense program at a cost of trillions of yen.

Tokyo is most concerned about North Korea's missiles, which are believed capable of reaching Japan within 10 minutes of being fired.