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SEOUL, South Korea (AP) — Recent satellite images show North Korea is digging a new underground tunnel in what appears to be preparation for a third nuclear test, according to South Korean intelligence officials.

The excavation at North Korea's northeast Punggye-ri site, where nuclear tests were conducted in 2006 and 2009, is in its final stages, according to a report by intelligence officials that was shared Monday with The Associated Press.

Its release comes as North Korea prepares to launch a long-range rocket that Washington and others say is a cover for testing missile technology that could be used to fire on the United States.

Observers fear a repeat of 2009, when international criticism of the North's last long-range rocket launch prompted Pyongyang to walk away from nuclear disarmament negotiations and, weeks later, conduct its second nuclear test. A year later, 50 South Korean were killed in attacks blamed on the North.

"North Korea is covertly preparing for a third nuclear test, which would be another grave provocation," said the report, which cited U.S. commercial satellite photos taken April 1. "North Korea is digging up a new underground tunnel at the Punggye-ri nuclear test site, in addition to its existing two underground tunnels, and it has been confirmed that the excavation works are in the final stages."

Dirt believed to have been brought from other areas is piled at the tunnel entrance, the report said, something experts say is needed to fill up underground tunnels before a nuclear test. The dirt indicates a "high possibility" North Korea will stage a nuclear test, the report said, as plugging tunnels was the final step taken during its two previous nuclear tests.

North Korea announced plans last month to launch an observation satellite using a three-stage rocket during mid-April celebrations of the 100th anniversary of the birth of North Korean founder Kim Il Sung.

The U.S., Japan, Britain and other nations have urged North Korea to cancel the launch, warning that firing the long-range rocket would violate U.N. resolutions and North Korea's promise to refrain from engaging in nuclear and missile activity.