The North Koreans were contacted through their United Nations mission in New York, said US Assistant Secretary of State Christopher Hill, the lead negotiator to the six party talks aimed at ending Pyongyang's nuclear weapons program.
"Yesterday we sent the message directly to the DPRK (North Korea) through the New York channel on our view of what such a test would mean," Mr Hill said.
Asked to explain what he meant by the US view that was conveyed to North Korea, Mr Hill said a nuclear blast "would be a very highly provocative act, and the international community cannot be indifferent to that".
"It would invite the prospect of proliferation ... and we have no choice but to act resolutely to make sure the DPRK and every other country understand" the implications of such a move, Mr Hill said.
It’s believed to be the first direct contact between the two countries since North Korea announced that it was planning its first nuclear test explosion.
Mr Hill left the door open for direct negotiations to end the crisis.
"We have spoken consistently that we will meet them bilaterally, but has to be in the context of the six party talks," he said at the launching of the US-Korea Institute at the Johns Hopkins School of Advanced International Studies in Washington.
North Korea, Mr Hill said, should reconsider if it believed that blasting a nuclear device would make them a de facto internationally recognised atomic power.
“We are not going to wait for a nuclear North Korea, we are not going to accept it," he said.
North Korea, he said, had come to "a very important fork in the road - it can have a future or it can have these (nuclear) weapons but it cannot have them both".
Mr Hill said the six party talks involving China, Japan, the two Koreas, Russia and the United States was still the most viable channel to resolve the crisis.
North Korea walked away from the talks in November last year after Washington imposed financial sanctions against it.