South Korea has announced a deal with the U.S. to almost triple the range of its ballistic missile system, saying this will counter the threat from North Korea, BBC News reported.

The whole of the North, along with parts of China and Japan, will be within South Korea's new 800km range.

South Korea had been restricted to missiles with a range of 300km under a previous security deal.

It was revised after North Korea tested a long-range rocket launch in April.

The U.S. has more than 28,000 troops in South Korea and provides security guarantees for its ally.

The new deal puts North Korean military facilities which were previously out of range within reach, as well as parts of China and Japan.

It will also allow South Korea to triple the payload from 500kg for missiles over shorter distances.

Three weeks ago, the U.S. and Japan agreed to set up a second missile defence system on Japanese soil in response to the North Korean threat.