The United States says it's committed to defending Japan and opposes any coercive action by China to seize territory under Japanese control in the East China Sea.
Secretary of State John Kerry and his South Korean counterpart say North Korea will gain nothing by threatening tests of its missile or nuclear programs.
President Barack Obama says now is the time for North Korea to end its belligerence. He says the United States will take, in his words, "all necessary steps" to protect its people.
Experts say North Korea is probably years away from perfecting the technology to back up its bold threats of a pre-emptive nuclear strike on America, but some aren't sure about nearby targets like South Korea and Japan.
The North Korean army is warning Washington that its military has been cleared to wage an attack using "smaller, lighter and diversified nuclear" weapons.
North Korea says it has entered `a state of war' with South Korea in the latest of a string of threats that have raised tensions on the Korean Peninsula.