Tokyo on Monday cautiously endorsed South Korea's expanded air defence zone, saying it had blasted a similar move by China because it covered Japanese territory.
Regional tensions have been on high alert since Beijing declared an Air Defence Identification Zone (ADIZ) in the East China Sea last month, in which foreign planes are supposed to file flight plans with China.
The US, Japan and South Korea accused China of unilaterally changing the status quo and flew military and paramilitary aircraft into the area in shows of defiance.
On Sunday, South Korea announced an expanded air defence zone, which covers a submerged rock disputed by Beijing and Seoul, and said it would go into effect on December 15.
Japan's Chief Cabinet Secretary Yoshihide Suga on Monday said Seoul had informed Tokyo in advance about its plans - something Beijing had not done.
“We don't think it's going to be a problem at the moment,” Suga, the government's top spokesman, told reporters in Tokyo.
“It is different from the one announced by China because it does not cover our country's territorial air, waters or land,” he added.
Conservative Japanese Prime Minister Shinzo Abe told Defence Minister Itsunori Onodera to set up a “thorough system of communication” between Tokyo and Seoul over the expanded zone, Jiji Press news agency reported.
Seoul expanded its ADIZ by about 65 000 square kilometres - or about two thirds of the size of the country - in waters off its south coast, the defence ministry said.
The Chinese zone covers disputed Tokyo-controlled islands, known as the Senkaku islands in Japan and Diaoyu in China, that have been at the centre of a simmering territorial row. - Sapa-AFP