WASHINGTON — North Korea appeared to conduct its third, and probably largest, nuclear test on Tuesday, according to U.S. and Asian officials, posing a new challenge for the Obama administration in its effort to keep the country from becoming a full-fledged nuclear power.

North Korea state media said the military successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday, defying U.N. Security Council orders to shut down atomic activity or face more sanctions and international isolation.

The underground explosion could take North Korea a big step closer to its goal of building a nuclear warhead small enough to be mounted on a long-range missile that could threaten the United States.

North Korea faced sanctions after a December launch of a rocket the U.N. and the U.S. called a cover for a banned missile test.

The North said it used a “lighter, miniaturized atomic bomb” that still has more explosive force than past tests.

The U.S. Geological Survey said it was only 1 kilometer underground, an indication consistent with a nuclear blast. And in Vienna, the organization that monitors the Comprehensive Nuclear Test Ban Treaty said the tremor had “clear explosion-like characteristics.”

Preliminary estimates suggested a test far larger than the previous two conducted by the North, though probably less powerful than the first bomb the United States dropped on Japan, in Hiroshima, in 1945.

If confirmed, the test would be the first under the country’s new leader, Kim Jong Un, and an open act of defiance to the Chinese, who urged the young leader not to risk open confrontation by setting off the weapon.

The apparent test set off a scramble among Washington’s Asian allies to assess what the North Koreans had done.

The United States sent aloft aircraft equipped with delicate sensors that may, depending on the winds, be able to determine whether it was a plutonium or uranium weapon. The Japanese defense minister, Itsunori Onodera, said Japan sent a jet to monitor for radioactivity in Japanese airspace.

Iran is also pursuing uranium enrichment, and one senior U.S. official said two weeks ago that “it’s very possible that the North Koreans are testing for two countries.”