How North Korea's Nuclear Threats Have Spiraled Over The Decades

North Korea caused international concern and condemnation early this year when it claimed to have successfully detonated a nuclear hydrogen bomb yielding a 5.1 preliminary-magnitude earthquake in the town of Sungjibaegam.

Supreme leader Kim Jong-un said on state television the test would "make the world ... look up to our strong nuclear country."

The world is watching. While many experts are skeptical  of Pyongyang's unverified claim, others warn it should still be taken seriously .

Here's what we know about North Korea's nuclear program.

Early stages

North Korea is believed to have conducted four nuclear tests in the past decade. Its ambition to harness nuclear power can be traced back to the 1950s , when the Soviet Union started teaching North Korean scientists and engineers how to develop nuclear power.

In 1959, North Korea signed a nuclear cooperation agreement with the Soviet Union, which provided Pyongyang with basic nuclear training and technology for the following three decades.

Although the Soviet Union helped train North Koreans, it refused to help them create weapons of mass destruction. North Korea also asked  China for help developing nuclear weaponry in the 1960s, but its pleas were rejected.

It acceded in 1985 to the United Nations' Treaty on the Non-Proliferation of Nuclear Weapons , or NPT, which strives for nuclear disarmament, but never came into compliance. North Korea withdrew from the NPT in 2003, following failed negotiations with the U.S. that sought to freeze Pyongyang's nuclear program.

2006

North Korea conducted its first-ever nuclear test on Oct. 9, 2006. Later that day, the U.N. Security Council held an emergency meeting on the matter. All five veto members condemned the nuclear test. The council later imposed sanctions on North Korea, including an embargo on military and technological materials.

On Oct. 10, NATO issued a press release  condemning the test. "This test poses an extremely serious threat to peace and security in the Pacific region and the world," the statement said.

U.S. officials detected some radioactive debris in tests that followed the blast, which measured less than 1 kiloton , but suggested the device may have misfired. "The betting is that this was an attempt at a nuclear test that failed," a senior U.S. administration official said .

2009

North Korea conducted its second underground nuclear test on May 25, 2009. It was more powerful than the first, measuring 2.35 kilotons . Weeks later, the Security Council unanimously adopted Resolution 1874 , which extended existing economic and commercial sanctions  on North Korea to cover all arms material and related financial transactions, technical training, advice, services or assistance, manufacture and maintenance.

The test came on the heels of then-leader Kim Jong-Il's severe stroke. North Korea experts believe  the communist nation conducted the nuclear test to show its citizens and the world that it would remain a nuclear power, even during a time of leadership transition.

2013

Pyongyang said it carried out a third nuclear test on Feb. 12, 2013; however, investigators failed to detect any radiation. The explosion was approximately twice as large as the 2009 detonation, and the secluded country said it used a miniaturized nuclear device  with a greater explosive force. Experts believe it measured between 4 and 15 kilotons . 

After the nuclear and subsequent missile tests, the U.N. further tightened sanctions against North Korea. Japanese Prime Minister Shinzo Abe said the test was a "grave threat" that could not be tolerated.

North Korea announced in April  it was on the verge of a nuclear war with its neighbors in South Korea and warned foreigners to leave the country.

2016

Pyongyang's claim it has successfully tested a thermonuclear bomb continues to cause alarm and debate.

"The test means a higher stage of the DPRK's development of nuclear force," the North Korean government boasted in a statement  issued on Jan. 6, using the acronym for the country's official name, the Democratic People's Republic of Korea.

Some experts suggested a hydrogen bomb would have yielded a far more powerful seismic disturbance, but January's explosion was not significantly larger than the previous three. 

Nevertheless, North Korea's nuclear ambitions are still provoking international concern. U.N. Secretary-General Ban Ki-moon called North Korea's latest nuclear test  "profoundly destabilizing for regional security." North Korea's neighbor China also condemned the test , urging the nation to remain committed to denuclearization efforts.

Amid concern that North Korea could use space technology to enhance its missile capability, U.S. officials told Reuters  in late January they noticed increased activity around a North Korean missile site, suggesting preparations for a possible space launch could be forthcoming. Days later, Pyongyang alerted  the International Maritime Organization it planned to launch a rocket with a satellite attached in the month of February. 

Pyongyang made good on its promise and announced on Feb. 7 it had successfully launched satellite Kwangmyongsong-4. The move triggered strong international condemnation as countries including South Korea and the United States said the launch was a disguised missile test that violated U.N. sanctions. 

At an emergency meeting later that day, the members of the U.N. Security Council described the launch as "a clear threat to international peace and security," though no new sanctions were imposed. 

UPDATE: This article has been updated to include information about North Korea’s Feb. 7 satellite launch, which many countries condemn as a disguised missile test.