A senior US diplomat has warned North Korea against a nuclear test, saying that it would be a provocative act.
Mr Hill says ties with Pyongyang are in a "difficult period"
Nuclear negotiator Christopher Hill made the comments in Shanghai at the end of a six-day visit to China.
He also said North Korea would receive no further incentives to return to multilateral talks on its nuclear ambitions.
Mr Hill now flies to South Korea amid talks of a split between Washington and Seoul on how to handle Pyongyang.
Last week, South Korean President Roh Moo-hyun appeared to soften his stance, calling Pyongyang's recent missile tests "too meagre" to reach the US and "too big" to target South Korea.
Mr Roh is scheduled to fly to the US on Thursday for a meeting with President George W Bush.
Mr Hill arrived in the region last Monday acknowledging that there was no indication Pyongyang was seeking a return to talks. Ties were in a "very difficult period", he said.
On 5 July Pyongyang tested seven missiles, including a new long-range weapon capable of hitting the US. There has also been speculation that North Korea may be planning what is thought to be its first nuclear test.
Mr Hill warned again against such as step.
Mr Roh meets Mr Bush in Washington later this week
"With regards to a nuclear test, obviously this would be a very provocative act," he said. "The international community will not in any way support these provocative actions."
He urged North Korea to return to negotiations, saying there were no further incentives on the table.
"There is no reason why the other five (nations) should be sitting around looking for inducements to get the North Koreans to accept what is on the table, which is clearly in their interest to accept," he said.
Mr Hill said North Korea had to keep to a deal agreed at the most recent round of six-nation talks in September 2005.
The deal, which promised economic aid in return for Pyongyang scrapping its nuclear ambitions, appears to have fallen apart over disagreements on implementation.
"We're asking the North Koreans to come to the table and implement what they already agreed to do," he said.
Mr Hill now flies to Seoul for further talks ahead of a summit between the US and South Korean presidents on Thursday.
According to South Korean media, the two sides are apart on how to approach North Korea, with the US reported to be considering further economic sanctions targeting Pyongyang.
Last week, a senior South Korean official held talks in Washington amid media speculation of divisions.
South Korean Foreign Ministry officials confirmed there were no plans to issue a joint statement after the summit, Yonhap news agency reported.