North Korea said it possessed "strategic rocket forces" capable of striking the US mainland, as it responded to a new US-South Korean deal to extend the range of the South's missile systems.

In a series of bulletins released on the official Korean Central News Agency (KCNA) on Tuesday, the spokesman of the National Defense Commission also said Pyongyang was ready to match any enemy, "nuclear for nuclear, missile for missile".

The warnings came two days after South Korea announced an agreement with the United States to almost triple the range of its missiles to cover the whole of North Korea.

"We are not concealing the fact that (North Korea's) revolutionary military, including strategic rocket forces, has placed not only South Korean enemy forces and US forces in the Korean peninsula but also Japan, Guam and even the US mainland within its target range," the spokesman said.

North Korea is known to have an inter-continental ballistic missile in development -- the Taepodong-2 -- but it has never been tested successfully. In April, the North carried out a failed rocket launch in what it said was a bid to put a satellite into orbit. The US and United Nations condemned the launch as a disguised ballistic missile test, saying the rocket was simply a three-stage variant of the Taepodong-2.