The country defied U.N. warnings and carried out the atomic test Monday. North Korea called it a "first response" to what its leaders called threats from the U.S.
The U.N. Security Council and the Defense Department pledged further action against the country.
Secretary of Defense Leon Panetta said, "We just saw what North Korea has done in these last few weeks, a missile test and now a nuclear test. They represent a serious threat to the United States of America, and we've got to be prepared to deal with that."
The test will likely draw more sanctions against North Korea.