Christopher Hill, the lead US negotiator on the North's nuclear programme, arrived in South Korea for talks on ways to enforce United Nations sanctions.
"Now we have the evidence to confirm what they have done," he told reporters at the airport.
"This does not change, in our view, what we have to do -- which is to work very hard with our partners and allies to implement the UN Security Council Resolution 1718.
"North Korea will pay a very, very high price for this type of reckless behaviour."
The United States said on Monday that tests of air samples confirmed that North Korea conducted a nuclear test on October 9, but the test had an explosion yield of less than one kiloton.
Japan's foreign minister Taro Aso and media reports said the communist state may be preparing a second nuclear test. Hill said he had seen the reports but had no further information.
Hill is paving the way for a visit to Seoul on Thursday by US Secretary of State Condoleezza Rice. She will hold talks with Aso and her South Korean counterpart Ban Ki-Moon.
Hill said the scheduled meeting "reflects very much what we are trying to do. And that is to have intense diplomatic activity aimed at looking for all ways we can find to isolate the North Koreans and increase the cost of this (nuclear) programme."
The Security Council, which imposed sanctions on the North following its July missile tests, broadened those measures in Saturday's new resolution -- demanding the elimination of all nuclear weapons, weapons of mass destruction and ballistic missiles.
It provides for inspection of cargo to and from North Korea, a travel ban on officials working on missile or nuclear programs, and a ban targeting missiles, tanks, large artillery systems, warships and combat aircraft.
Hill said the US's ultimate goal is not to impose sanctions.
"Our goal here is to get North Korea to implement the joint statement and to get out of the business of nuclear weapons and nuclear programmes."
He urged Pyongyang to look again at the incentives offered under a joint statement in September 2005 at six-party talks. In the statement, the North agreed to scrap its nuclear programs in exchange for energy and economic benefits and security guarantees.
Hill was chief US delegate to the talks, which also group the two Koreas, China, Russia and Japan. The North has boycotted the forum since November in protest at US financial restrictions.

Copyright AFP (Agence France-Presse), 2006