North Korea's surprise announcement this week that it had tested a powerful hydrogen bomb was met with doubt, condemnation and questions on so-far failed efforts to stop the country's growing nuclear threat. Here's a look at some of the key developments since North Korea announced its fourth nuclear test Wednesday.

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WEDNESDAY, JAN. 6

— Seismic stations in the U.S., Europe and South Korea detect an earthquake near an area where North Korea had conducted its three previous nuclear tests. Seoul's agency says the 4.8-magnitude quake was likely caused by a manmade explosion. North Korea's state TV announces hours later that scientists have achieved "perfect success" in testing a "miniaturized" H-bomb.

— Seoul's spy agency quickly disputes North Korea's claim, saying that the seismic event was too small for an H-bomb, which is dramatically more destructive than an atomic bomb, but also much more difficult to make.

— North Korea's announcement draws strong condemnation from governments around the world. South Korean President Park Geun-hye orders her military to bolster its combined defense posture with U.S. forces. China, Pyongyang's most important economic and strategic partner, urges the North to refrain from acts that might worsen tensions on the Korean Peninsula.

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THURSDAY, JAN. 7

— The U.N. Security Council holds an emergency meeting and pledges to swiftly pursue new sanctions against North Korea. It strongly condemns the nuclear test as a "clear violation" of previous U.N. resolutions.

— U.S. President Barack Obama speaks to the leaders of South Korea and Japan. The White House says the countries vow to work together to "forge a united and strong international response to North Korea's latest reckless behavior."

— Seoul's Defense Ministry says the military leaders of South Korea and U.S. discuss the deployment of U.S. "strategic assets," likely referring to nuclear-powered submarines and warplanes, in the wake of the North's nuke test.

— South Korea decides to retaliate over the nuke test by restarting from Friday anti-Pyongyang propaganda broadcasts along its border with the North for the first time in nearly five months. The broadcasts previously drew a furious response from the North and, according to Seoul, touched off an exchange of artillery fire. Seoul turned off its speakers in August after marathon negotiations with the North. Seoul also limits the entry of some South Koreans to an industrial park in North Korea jointly run by the two Koreas, which has been a valuable cash source for the North.

— At least three U.S. intelligence-gathering aircraft, including an RC-135S, which is capable of collecting optical and electronic data from ballistic targets, depart from an American air base in southern Japan, presumably to determine what kind of nuclear device the North detonated. White House spokesman Josh Earnest tells reporters that the U.S. will be "collecting additional evidence" and working closely with other countries in the region to learn more about the North Korean test.

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FRIDAY, JAN 8

— South Korea proceeds with plans to restart anti-Pyongyang propaganda broadcasts across the border on Friday, believed to be the birthday of North Korean leader Kim Jong Un. The broadcasts include popular South Korean pop songs as well as news and comments about Seoul's economic affluence and democracy, South Korean military officials say.

— U.S. Secretary of State John Kerry tells reporters he spoke with Chinese Foreign Minister Wang Yi over the phone and urged Beijing to end "business as usual" with North Korea after its nuke test.