U.S. broadcaster NBC says North Korea has notified China of its plan to conduct a series of underground nuclear tests, amid media reports on increasing signs of a second test.

NBC on Wednesday quoted U.S. officials as saying that intelligence satellites have detected unusual movement of vehicles and personnel that indicate North Korea may be preparing another nuclear test.

Meanwhile, Vice Foreign Minister Lee Kyu-hyung denied that the Chinese government was notified of a second test.

North Korea had notified China of its first nuclear test on October ninth and Beijing delivered the information to Washington and Seoul.

Quoting a U.S. intelligence official, CNN earlier reported that senior North Korean military officials said they plan to conduct "multiple tests" and that U.S. spy satellites have detected movements that could be preparations for nuclear tests at "as many as three sites."

Reuters quoted a senior U.S. official as saying that a second test by the North is probably imminent.

In Washington, White House spokesman Tony Snow said that if Pyongyang carries out a second test, it will face further isolation from the international community.

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