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North Korea carried out its latest nuclear test hours before President Barack Obama planned a major speech, renewing speculation that it times major incidents with the United States in mind.
North Korea, issuing a statement via its official media on its third explosion of a nuclear device, said that it was responding to "hostility" from the United States.
The test came less than 24 hours before Obama -- who has mostly tried to ignore North Korea for want of better options -- speaks to a joint session of Congress for his annual State of the Union address.
Obama issued a late-night statement on the eve of his address, calling the test "provocative" and saying that North Korea constituted "a threat to US national security and to international peace and security."
North Korea carried out its first two tests on US holidays when most of official Washington was closed. Its first test took place on Columbus Day in October 2006 and its second explosion was on Memorial Day in May 2009.
The communist regime also carried out a test of what were believed to be seven ballistic missiles, including the long-range Taepodong-2, on the US Independence Day in July 2006.
However, other factors could also potentially explain the timing. February 16 is the birth anniversary of Kim Jong-Il, the longtime leader who died in 2011.
North Korea regularly choreographs events for such key dates. It carried out an unsuccessful rocket launch in April last year around what would have been the centennial of the birth of regime founder Kim Il-Sung.
The latest test comes days before South Korea, which remains technically at war with the North, inaugurates Park Geun-Hye as president on February 25. Park had pledged to find new ways to engage North Korea.
The test also falls amid the region's Lunar New Year celebration, when much of China is shut down. China is North Korea's closest ally and its official media had been unusually forthright in warning against a test.
Some North Korea watchers caution not to read too much into the timing of tests, believing that the regime is primarily driven by internal politics due to the dominance of its military.