Mr Kelly made the remarks on a visit to Beijing after Pyongyang described Washington's offer to revive aid in exchange for the Communist state dropping its nuclear programme as "deceptive drama".
"
The US talk about dialogue is nothing but a deceptive drama to mislead the world public opinion
"
N Korean Foreign Ministry statement
In a statement carried by North Korea's official news agency, the Foreign Ministry said the confrontation would only end when the US signed a non-aggression pact.
The announcement appeared to dash fresh hopes of a conclusion to the crisis which were raised on Tuesday after President George W Bush spoke of a "bold initiative" of benefits for North Korea.
Mr Bush's overture signalled that the US might be interested in reopening discussions with North Korea for the first time since the crisis erupted three months ago.
Observers have suggested that North Korea has simply been hoping all along to force the US into providing fresh aid.
Correspondents say that Pyongyang's apparent defiance of the latest US offer should not necessarily be taken as a clear rejection of it.
The White House said the KCNA statement was "unfortunate", but added that it would await an official response from Pyongyang.
"
The use of force should be the last means in the
arsenal of measures which can be used
"
Mohammed ElBaradei
IAEA
"North Korea has a habit of saying very many inflammatory things, and then, even in their inflammatory things can sometimes contradict themselves and so can their private statements," said spokesman Ari Fleischer.
Washington has also denied a report by Japan's Kyodo news agency that the US had written to North Korea offering a guarantee that it would not attack.
A non-aggression pact from Washington would be unlikely, South Korea's defence minister said on Thursday.
"Because this could signal an intention... to expand the debate to the withdrawal of US troops from South Korea, the United States will not easily accept it," Lee Jun told parliament.
Mr Lee also admitted that Seoul was prepared for the "worst case scenario" on the Korean peninsula.
Diplomatic efforts
North and South Korea have agreed however to hold high-level talks in Seoul next Tuesday - the first such meeting since the North announced it was withdrawing from a key nuclear treaty last week.
Mr Kelly has meanwhile been holding talks with China - a long time ally of North Korea - which he described as "very good".
CRISIS CHRONOLOGY
"The Korean peninsula needs to be free of nuclear weapons, that's something China, the USA, South Korea, Japan and Russia and the whole international community agrees on," Mr Kelly said.
"It's going to be a slow process to make sure we achieve this in the right way," he added.
Beijing has proposed hosting talks between North Korea and the US while Russia, another state with close ties to Pyongyang, has also offered its help.
To that end, the head of the International Atomic Energy Agency (IAEA), Mohamed ElBaradei, has begun talks in Moscow.
North Korea ejected the last IAEA inspectors on New Year's Eve as part of its move to revive its nuclear programme, which is suspected by the West of being intended for arms production.
Moscow has already put forward its own plan to resolve the crisis and Russia's Deputy Foreign Minister, Alexander Losyukov, is expected to travel to Beijing on Friday to discuss it.