Japan has ordered aircraft to report to the US military first if they enter airspace near the main US military base in Okinawa, a report said early Saturday after fresh North Korean threats.
The transport ministry notice issued late Friday was believed to be part of precautions against possible North Korean missile launches, Kyodo News agency reported.
The order refers to airspace within 30 kilometres (20 miles) of the Kadena base where Patriot missiles have been deployed.
It comes after North Korea threatened that Japan would be "consumed in nuclear flames" if it got involved in any conflict on the Korean peninsula,
Japan, the only country ever to have suffered a nuclear attack, has authorised its armed forces to shoot down any North Korean missile headed towards its territory.
This week Patriot missile batteries were stationed around Tokyo to protect the 30 million people who live there.
In addition to the Patriots, Aegis destroyers equipped with sea-based interceptor missiles have been deployed in the Sea of Japan (East Sea) and on Friday the government said it would be permanently installing missile defences in Okinawa.