New satellite images suggest North Korea is digging a new underground tunnel in preparation for a nuclear test this month.

The communist nation plans to launch a long-range rocket sometime between April 12 and 16.

A State Department spokesperson couldn't confirm if the North was preparing for its third nuclear test but said it would only further isolate the government.

The United States, Britain, Japan and other countries have urged North Korea to cancel the launch, saying it would be considered a violation of U.N. resolutions.

Meanwhile, a committee for human rights in North Korea is holding a conference in Washington today.

The conference is timed to coincide with Pyongyang's centennial celebrations of its repressive founder, President Kim Il Sung.

The group said satellite images show more than 150,000 North Koreans are imprisoned in Soviet-style, hidden labor camps known as gulags.