US Secretary of State Condoleezza Rice has warned North Korea that a second nuclear test would be a provocative act that could only deepen its isolation.
She was speaking as she prepared for a tour of Asia to bolster the sanctions the UN has imposed on North Korea.
The US says it has detected movement at last week's test site but cannot confirm if it points to a second test.
US intelligence officials said on Monday that air samples had confirmed the blast on 9 October was nuclear.
After the blast, the UN Security Council unanimously voted to impose sanctions, targeting Pyongyang's weapons and missile programmes as well as luxury goods.
Inspections
Ms Rice, who leaves on Tuesday for Japan, South Korea, China and Russia, said UN powers must honour "obligations" to punish the North.
"We expect every member of the international community to fully implement all aspects of this resolution. And we expect the Security Council to aggressively monitor the process," she said.
NEW UN SANCTIONS
"As North Korea scorns the international community, we will collectively isolate North Korea from the benefits of participation in that community."
She warned that a second test by the North would "further deepen the isolation of North Korea and I hope they would not take such a provocative act".
A White House official said US spy satellites had detected suspicious movements near the 9 October blast site but these could not be confirmed as relating to a second test.
Japanese Foreign Minister Taro Aso said on Tuesday Tokyo was aware of the reports but that he could not disclose details.
The South Korean government also said it was analysing the intelligence reports.
US envoy Christopher Hill has now arrived in Seoul for talks on how to enforce the UN sanctions.
Ms Rice said she was confident China would not "turn their backs on their obligations" amid reports of Beijing's concern over some sanctions inspections.
Beijing's envoy to the UN Wang Guangya appeared to rule out stopping North Korea ships at sea, known as interdiction.
But US Undersecretary of State Nicholas Burns said there were signs Beijing was implementing the sanctions.
"We have some indications that the Chinese are stopping trucks and inspecting them across that 800-mile border this morning," he said.
Ms Rice accepted there were "natural concerns" that seizing cargoes could spark conflict and said her trip would discuss "mechanisms" for implementing the sanctions.
Small blast
US Director of National Intelligence John Negroponte said on Monday findings had shown North Korea did carry out a nuclear explosion a week ago.
"North Korea cannot endanger the world and then expect other nations to conduct business as usual"
A US military aircraft collected air samples on 11 October, two days after Pyongyang made its announcement of a successful test.
The apparently small size of the explosion had led to doubts over the veracity of North Korea's claim.
But the short statement from Mr Negroponte's office confirmed that a nuclear explosion with a yield of "less than a kiloton" took place.
This is less than a tenth of the size of the bomb dropped on Hiroshima in 1945.
Russia had previously been the only country to confirm the test.
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