North and South Korea have been technically at war since their 1950-53 conflict ended in a truce, not a peace treaty, and regional powers have for years been trying to rein in North Korea's nuclear weapons program.

Reclusive North Korea is believed to be developing a long-range missile with a range of 6,700 km (4,160) miles) or more aimed at hitting the United States, but two recent rocket tests failed.

Its neighbors fear North Korea is using rocket launches to perfect technology to build a missile capable of delivering a nuclear warhead to the United States.

North Korea's National Defence Commission said in a statement that the country was prepared to counter any U.S. military threats, its KCNA news agency said.

"We do not hide (the fact) that the revolutionary armed forces ... including the strategic rocket forces are keeping within the scope of strike not only the bases of the puppet forces and the U.S. imperialist aggression forces' bases in the inviolable land of Korea, but also Japan, Guam and the U.S. mainland," KCNA said.

The U.S. State Department declined to discuss whether it believed North Korea's new claims on missile range, saying this was an intelligence issue. But it noted that North Korea is bound by U.N. Security Council resolutions to suspend all activities related to ballistic missile programs.

"Certainly rather than bragging about its missile capability, they ought to be feeding their own people," State Department spokeswoman Victoria Nuland said, adding that "threats or provocations" by North Korea would only undermine its efforts to seek more engagement with the international community.

South Korea on Sunday unveiled an agreement with the United States that extends the range of its ballistic missiles by more than twice its current limit to 800 km (497 miles) as a deterrent against North Korea.

North Korea is under heavy U.N. sanctions that have cut off its previously lucrative arms trade and further isolated the state after its failed 2009 missile test drew sharp rebukes, even from its one major ally, China.

The United States has denied it has any intention to strike North Korea. It has more than 20,000 troops stationed in South Korea in defense of its ally against North Korea.

In April, under its new leader Kim Jong-un, North Korea again launched a rocket that flew just a few minutes covering a little over 100 km (60 miles) before blowing up over the sea between South Korea and China.