Air samples from North Korea confirm that a nuclear explosion was carried out a week ago, US intelligence officials say.
Pyongyang announced it had carried out a nuclear test on 9 October
Director of National Intelligence John Negroponte said the findings came after analysis of radioactive debris detected at the site of the test.
This the first official US confirmation that a nuclear detonation took place.
The UN Security Council has unanimously voted to impose sanctions against Pyongyang in response to the test.
Shortly after the confirmation, US Secretary of State Condoleezza Rice warned North Korea against conducting a second test.
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, saying within only a few hours of North Korea's announcement that it was "100% certain" a nuclear test had been carried out.
Ms Rice, who leaves on Tuesday for talks in Japan, South Korea and China, told reporters that she hoped Pyongyang would not conduct a second test.
"That would further deepen the isolation of North Korea, and I hope they would not take such a provocative act," she said.
Ms Rice said that during her trip she planned to reassure leaders in Tokyo and Seoul that the US would stand by its security commitment to them.
The confirmation of the test comes as key nations try to agree on how to implement the sanctions agreed by the Security Council on Saturday.
NEW UN SANCTIONS
Bans sale to, or export from, N Korea of military hardware
Bans sale or export of nuclear and missile related items
Bans sale of luxury goods
Freezes finances and bans travel of anyone involved in nuclear, missile programmes
Allows inspection of cargo to and from N Korea
Stresses new resolution needed for further action
The measures target Pyongyang's weapons and missile programmes, as well as luxury goods.
They also permit cargo coming from or going to North Korea to be inspected for banned items.
But disagreements have emerged between the members of the Security Council over this part of the resolution, with China concerned that inspecting cargo at sea could further elevate tensions and lead to naval clashes.
Top US officials have urged China to comply with the resolution, and US Undersecretary of State Nicholas Burns told American television that there were signs Beijing was implementing the sanctions.
"We have some indications that the Chinese are also stopping trucks and inspecting them across that 800-mile border this morning," he said.
But Beijing's envoy to the UN Wang Guangya appeared to rule out stopping North Korea ships at sea, known as interdiction.
"Inspections yes, but inspections are different from interception and interdiction," he told journalists. "I think different countries will do it in different ways."
US envoy Christopher Hill, meanwhile, has arrived in Tokyo for talks on how to enforce the sanctions. He will also hold talks in South Korea.
Japan has already banned North Korean ships from its ports, as has Australia, and is looking at whether it can provide logistical support for US vessels if they start trying to inspect cargo ships going to or from North Korea.