A deal between the United States and South Korea to almost triple the range of South Korean missiles to cover all of North Korea will help Seoul defend itself from attack, a US official said Tuesday.

"These revisions are a prudent, proportional, and specific response to the DPRK ballistic missile threat," a State Department spokesman told AFP, referring to North Korea, also known as the Democratic People's Republic of Korea (DPRK).

The new guidelines "are designed to improve the (South's) ability to defend against DPRK ballistic missiles," the official said in an email, adding world leaders had long voiced concern over Pyongyang's missile program.

South Korea on Sunday announced the deal with the United States under which it can deploy missiles with a range of 800 kilometers (500 miles), up from the current limit of 300 kilometers.

The extension will not only bring the whole of North Korea within reach of Seoul's rockets, but also parts of China and Japan.

North Korea on Tuesday slammed the new deal as a provocation for war, and hit back that it possessed rockets capable of striking the US mainland.

In a statement carried by the official Korean Central News Agency (KCNA), a spokesman for the North's National Defense Commission said Pyongyang had "strategic rocket forces" with a "scope of strike" that not only covered US and South Korean bases in South Korea, "but also Japan, Guam and the US mainland."

The US stations 28,500 troops in South Korea and guarantees a nuclear "umbrella" in case of any atomic attack. In return, Seoul accepts limits on its missile capabilities.

The US official said Washington had been "in discussions with the Republic of Korea, at its request, on ways to address the threat posed by DPRK ballistic missiles."

The new revisions "as well as additional improvements to alliance capabilities" had resulted from these talks.

"As partners whose alliance is a linchpin of stability in Northeast Asia, we take seriously our mission of maintaining stability on the Korean Peninsula and in the region. The United States remains firmly committed to our alliance and to the defense of" South Korea, the US official added.