The Democratic People's Republic of Korea (DPRK) on Friday announced a successful nuclear warhead explosion test, the fifth nuclear detonation and the second this year. Following is a chronology of the country's nuclear development.

Dec. 12, 1985 -- The DPRK joins the Nuclear Non-Proliferation Treaty (NPT).

March 12, 1993 -- The DPRK declares its withdrawal from the NPT, but it later allows inspectors of the International Atomic Energy Agency (IAEA) to inspect nuclear-suspected sites.

Oct. 21, 1994 -- The DPRK and the United States sign the Geneva agreement in which Pyongyang pledges to freeze its nuclear program in return for U.S. aid to build two light-water reactors.

December 2002 -- Washington declares to stop providing heavy oil to Pyongyang, which expels IAEA inspectors and removes monitoring equipment in retaliation.

Jan. 10, 2003 -- The DPRK officially withdraws from the NPT.

Sept. 19, 2005 -- A joint statement is announced after a marathon dialogue between the six parties: the DPRK, South Korea, China, the United States, Russia and Japan. Pyongyang pledges to abandon all nuclear weapons and relevant nuclear program, while Washington promises the normalization of relations with the DPRK, nonaggression against the country as well as economic assistance.

July 5, 2006 -- The DPRK test-fires a Taepodong-2 missile on U.S. Independence Day, but it appears to have failed.

Oct. 9, 2006 -- The DPRK conducts its first atomic bomb test in an underground facility.

Feb. 13, 2007 -- The DPRK returns to the six-party talks, agreeing to close its nuclear facility in exchange for normalized ties with the United States and economic assistance.

June 27, 2008 -- The DPRK destroys a water cooling tower at its main Yongbyon nuclear facility in a symbolic gesture.

Dec. 8-11, 2008 -- The DPRK walks out from the six-party talks due to disputes over how to verify its nuclear abandonment.

April 5, 2009 -- Pyongyang launches an Unha-2 rocket, which Seoul and the international community denounces as a test of ballistic missile technology.

May 25, 2009 -- The DPRK conducts its second nuclear test. Its explosive yield is estimated at 3-4 kilotons.

April 13, 2012 -- Another launch of an Unha-3 rocket is conducted but it fails. Seoul and Washington blast it as a cover-up for long-range missile test.

Dec. 12, 2012 -- The DPRK successfully launches a long-range Unha-3 three-stage rocket from the Sohae Space Center.

Feb. 12, 2013 -- A third nuclear detonation is carried out in its main Punggye-ri underground test site. It is the first atomic device test under top DPRK leader Kim Jong Un. Its explosive yield is estimated at 6-7 kilotons, with highly-enriched uranium believed to be used as nuclear material.

Jan. 6, 2016 -- The DPRK tests what it claims the first hydrogen bomb, the fourth of its nuclear detonations. The purported thermonuclear test triggers toughest-ever UN Security Council sanctions.

Feb. 7, 2016 -- Pyongyang launches a Kwangmyongsong long-range rocket.