Japanese Defense Minister Itsunori Onodera said he believed the visit would "contribute to the efforts" to resolve outstanding issues.

WSJ: Trip to Pyongyang Irks Tokyo's Allies

Defense Minister Itsunori Onodera said the government won't reveal information on the nationality of the submarines due to intelligence concerns.

WSJ: Japan on Alert Over Submarines Near Its Territorial Waters

Japan's Defence Minister Itsunori Onodera said his nation was on "high alert", with anti-missile defences deployed in Tokyo as a precaution.

BBC: South Korea raises alert with North to 'vital threat'

"The government is considering the extent of what can be disclosed, " Kyodo news agency quoted Japanese Defence Minister Itsunori Onodera as saying.

BBC: Asia

Defense minister Itsunori Onodera has said the government has no intention of revealing information on the nation the submarines belong to due to intelligence concerns.

WSJ: Japan on Alert Over Unidentified Submarines