Earlier in the day, an earthquake occurred in South Korea, prompting Seoul's suspicions that Pyongyang had performed a nuclear test.
"We are monitoring and continuing to assess the situation in close coordination with our regional partners," Kirby said.
"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 added.
In 2003, North Korea withdrew from the international Nuclear Non-Proliferation Treaty, a deal meant to prevent the making and use of nuclear weapons.
On December 10, North Korean leader Kim Jong Un said that Pyongyang had a hydrogen bomb and was ready to use it to protect its sovereignty.