North Korea carried out a “successful” hydrogen bomb test Wednesday morning, the communist country claimed.

The announcement came in a special announcement on state-run television.

A magnitude-5.1 earthquake had been detected at a North Korean nuclear test site that was suspected of being “man-made.”

A confirmed successful test would mark a big step toward Pyongyang’s goal of building a warhead that can be mounted on a missile capable of reaching the U.S. mainland.

This isn’t the first time North Korea has conducted a nuclear test. The last occurred in February of 2013.

State Department spokesman John Kirby said the U.S. was “monitoring and continuing to assess the situation in close coordination with our regional partners,” but could not independently verify the country’s claim of a successful test.

“While we cannot confirm these claims at this time, we condemn any violation of UN Security Council Resolutions and again call on North Korea to abide by its international obligations and commitments,” he said.

Kirby iterated that the U.S. “will continue to protect and defend our allies in the region” and “will respond appropriately to any and all North Korean provocations.”

Pyongyang is thought to have a handful of crude nuclear weapons. The United States and its allies worry about North Korean nuclear tests because each new blast brings the country closer to perfecting its nuclear arsenal.

The Associated Press contributed to this report. This story has been updated with a statement from the State Department.

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