Washington is "fully committed" to protecting Japan, US Secretary of State John Kerry said on Sunday, days after Pyongyang threatened the Japanese capital would be "consumed in nuclear flames".

"The United States is fully committed to the defence of Japan," Kerry told a joint press conference in Tokyo with Japan's Foreign Minister Fumio Kishida, ahead of a widely anticipated missile launch by North Korea.

Kerry's comments came after the Korean Central News Agency (KCNA) said on Friday that any attempt by Japan to shoot down a North Korean missile would result in war.

"Japan is always in the cross-hairs of our revolutionary army and if Japan makes a slightest move, the spark of war will touch Japan first," KCNA said in a commentary.

"Japan must come to its senses and behave rightly," it added.

Japan, the only country to have suffered a nuclear attack, has stationed anti-ballistic missile batteries in and around its heaving capital, and vowed it would respond.

"All we can say is we will take every possible measure to respond to any scenario," a spokesman for the defence ministry told AFP.