The threat was made on Thursday during a meeting between US Assistant Secretary of State James Kelly and North Korean officials.
They US officials noted, however, that the isolated, communist nation had made similar threats in the past.
"In the course of that discussion, the North Koreans made a reference to testing and they made it as part of an argument why we should accept their proposal right away," said one Bush administration official. "It was a fairly transparent ploy."
"It is similar to things they have said before. Threats that North Korea has made before have really gotten them nothing; they have only increased North Korea's isolation"
US administration official
He said the bulk of the meeting -described as "thorough and serious" - covered a US proposal floated on Wednesday to allow other nations to supply energy aid to North Korea if it agreed to the complete, verifiable and irreversible dismantling of its suspected nuclear programmes.
"It is similar to things they have said before. Threats that North Korea has made before have really gotten them nothing; they have only increased North Korea's isolation," the US official said. "The way out of that is not to threaten but rather to accept our proposals and resolve all these issues."
Another US official said the issue was raised in a low-key manner by North Korean Foreign Ministry officials who argued that they wanted a solution but hardliners in their Defence Ministry were less enthusiastic and so Washington should accept Pyongyang's proposal of a nuclear freeze for compensation.
Asked if the North Korean reference to testing could be viewed as a throwaway remark, one official said: "No North Korean reference to testing is a throwaway remark."
"Kelly again and again made the point that our proposal was the way to resolve all these (issues) and to get North Korea out of the hole that it has dug for itself," he added.