The United States has detected fresh signs that North Korea may be preparing for a second nuclear test, according to U.S. media reports Monday.

U.S spy satellites have detected "suspicious vehicle movements" that could be preparations for another test near the site where North Korea conducted its first underground nuclear explosion test on Oct. 9, ABC News said, quoting unidentified U.S. officials.

NBC News also reported unidentified U.S. officials as saying U.S. spy satellites are picking up signs of truck and people movements.

The news reports came after the United States officially confirmed earlier Monday for the first time that Pyongyang carried out a nuclear test on Oct. 9 in northeastern North Korea, thus also effectively confirming Pyongyang's possession of nuclear weapons.

Since its announcement Oct. 9 that it "successfully" conducted its first nuclear test, North Korea has repeatedly said it intends to take further "physical" actions against what it calls the "hostile" U.S. stance.

Washington called the North Korean test claim itself a "provocative" act, and led efforts in the U.N. Security Council to respond, which resulted in the unanimous adoption Saturday of a binding resolution requiring U.N. members to take economic and diplomatic sanctions against Pyongyang.

Earlier Monday, the U.S. National Intelligence Director John Negroponte's office issued a brief statement saying, "Analysis of air samples collected on Oct. 11, 2006 detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion in the vicinity of Punggye on Oct. 9, 2006."

"The explosion yield was less than a kiloton," the top U.S. intelligence office said. A kiloton nuclear explosion equals a blast of 1,000 tons of TNT.

But the office did not give any further details such as whether the test was a failure or a successful one as claimed by North Korea.