His remarks came as the US says it has deployed anti-missile defences around Hawaii, following reports that North Korea is preparing to fire its most advanced ballistic missile in that direction to coincide with July 4, the US independence day.
Last week, North Korea vowed to bolster its nuclear arsenal and threatened war to protest UN sanctions in the wake of its May 25 nuclear test, the country's second.
They conducted their first nuclear test in October 2006 and there are suspicions it is preparing for a third.
Robert Gates, the US defence secretary, said on Thursday that the US military has set up additional defences around Hawaii, consisting of a ground-based mobile missile system and a radar system nearby.
Together they could shoot an incoming missile in midair.
Gates spoke after Japan's Yomiuri newspaper reported that North Korea might test-fire a Taepodong-2 missile with a range of up to 6,500 km.
Yomiuri said the missile would fly over Japan but would not be able to reach Hawaii, which is at least 7,200km from the Korean peninsula.
North Korea test-fired a similar long-range missile on July 4 three years ago, but it failed seconds after liftoff.
Han Seung-soo, the prime minster of South Korea, said: "We hope that North Korea, first of all [will] give up nuclear ambitions and abide by the agreement that we made in 1992 - that is, a basic agreement for denuclearisation of the Korean peninsula."