Following is a review of the country's previous nuclear tests:

Oct 9, 2006 -- The official KCNA news agency reports that the country has conducted a successful nuclear test in North Hamgyong-do in the northeast of the country. The UN Security Council on Oct 14 passes Resolution 1718 to condemn the test.

May 25, 2009 -- The DPRK conducts its second nuclear test. The KCNA says that DPRK scientists have been able to increase the explosive power and control level of the bomb compared with the first test.

The test prompts UN Security Council Resolution 1874, which voices "the strongest condemnation" against DPRK authorities and demands the country stop further nuclear weapon-related activities.

Feb 12, 2013 -- The DPRK carries out a third nuclear test, which the KCNA describes as "high level, safe and perfect."

The ROK estimates the bomb has a power equal to 6,000-7,000 tons of TNT. The UN Security Council unanimously passes Resolution 2094 on March 7, demanding the DPRK give up its nuclear weapon project and return to the Nuclear Non-Proliferation Treaty.

Xinhua contributed to the story