U.N. responds with a sternly worded letter in 3… 2… 1.

(CNN) — North Korea appeared to have conducted its third underground nuclear bomb test Tuesday, as the U.S. Geological Survey reported a seismic disturbance centered near the site of the secretive regime’s two previous nuclear tests.

The area around the reported epicenter of the magnitude 4.9 disturbance has little or no history of earthquakes or natural seismic hazards, according to U.S. Geological Survey maps. The disturbance took place at a depth of about 1 kilometer, the USGS said.

There were no initial reports concerning the activity on the North’s state-run Korean Central News Agency on Tuesday.

“It’s a nuclear test,” said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies. “That magnitude and that location — it’s awfully unlikely it’s anything else.”

In Washington, a senior administration official said the United States was working to confirm a nuclear test.