Japan has deployed Patriot missiles in and around central Tokyo as a defensive measure should North Korea decide to launch an attack amid rising tensions in the Korean Peninsula.
Tokyo has moved three Patriot Advanced Capability-3 (PAC-3) surface-to-air missile launchers, according to the defence ministry. Two more batteries are likely to be stationed at other places.
The units are meant to defend the headquarters of the defence ministry and key military installations in the capital as well as the 30 million people who live in greater Tokyo.
"We are proceeding with measures including deployment of PAC-3 as we are on alert," said Tokyo's Defence Minister Itsunori Onodera.
He said another unit would be stationed in Okinawa prefecture as "the place that is most effective in responding to emergencies".
Although Japan has poured scorn on North Korea's threats thus far, the latest move is evidence that it could be preparing for the worst.
Tokyo had already deployed Aegis class destroyers equipped with missile interceptors in the Sea of Japan to counter the North Korean threats, and authorised its forces to shoot down any missile emanating from North Korea.
"The government is making utmost efforts to protect our people's lives and ensure their safety. As North Korea keeps making provocative comments, Japan, cooperating with relevant countries, will do what we have to do. For the moment, the most important thing is to implement sanctions under the UN Security Council resolutions," Japanese Prime Minister Shinzo Abe told reporters.
Further details about the deployment have been withheld by the authorities because of the sensitive nature of the issue.
"We'd like to refrain from explaining further because it would give away details of the cards we hold. At any rate, we have taken thorough measures to ensure the safety of the people," said Chief Cabinet Secretary Yoshihide Suga.
The announcement from Tokyo, Washington's close ally in the region, has come after Seoul-based reports suggested that North Korea has moved two medium-range missiles off its eastern coast.
While former North Korean leader Kim Il-Sung's birthday falls on 15 April, experts believe Pyongyang's current leadership may resort to provocative actions coinciding with the occasion to boost its own reputation domestically.
North Korea had earlier urged foreign diplomatic missions in the country to consider evacuating their staff before 10 April, citing the prospect of an imminent war.