The US State Department said North Korea's views, set out in a long statement on Tuesday, did not match the agreement signed in Beijing on Monday.
China asked all sides to fulfil their promises. Seoul said it would take the lead role in bridging the gap between the US and North Korean views. Japan saw a possible negotiating ploy.
"We must watch North Korea closely to see if there is really a fundamental difference on that point," Japanese chief cabinet spokesman Hiroyuki Hosoda said.
"If we are completely at odds, that will mean going back to the beginning. But we do not believe that is the case."
The six countries, also including Russia, had agreed on Monday to a set of principles on winding up Pyongyang's nuclear programme in return for aid and recognition of its right to a civilian nuclear programme.
"The US should not even dream of the issue of the DPRK's dismantlement of its nuclear deterrent before providing LWRs"
North Korea Foreign Ministry
The six agreed to discuss providing a light-water reactor "at an appropriate time".
But on Tuesday, the North Korea Foreign Ministry said in a statement published by the official KCNA news agency: "The US should not even dream of the issue of the DPRK's dismantlement of its nuclear deterrent before providing LWRs (light-water reactors).
"This is our just and consistent stand as solid as a deeply rooted rock."
DPRK is short for the North's official name, the Democratic People's Republic of Korea.
"It could be a lot of bluster," said one US official in Washington. But Tuesday's statement posed at least a challenge to a deal which delegates had applauded less than 24 hours earlier.
"This was obviously not the agreement they signed and we will see what the coming weeks bring," said US State Department spokesman Sean McCormack, referring to the gap before talks are set to resume in November. Japan took the same view.
"This was obviously not the agreement they signed and we will see what the coming weeks bring"
Sean McCormack, spokesman,
US State Department
Referring to its nuclear deterrent, North Korea's chief delegate, Vice Foreign Minister Kim Kye-gwan, told reporters in Beijing: "There will be no such thing as giving it up first."
Back in Pyongyang, he said he hoped the North and Washington could take real action on the Beijing agreement.
Seoul said the North's overall comments were no surprise.
"North Korea and the United States may pull and push over the wording of 'appropriate time' but the South Korean government's role is to mediate," President Roh Moo-hyun's spokesman quoted him as saying.
Roh said the deal should open the gates to more economic aid to the impoverished North.