An earthquake measuring 6.3 struck North Korea today, suggesting the reclusive country may have conducted a sixth nuclear test. The US Geological Survey said the quake struck 55 km north-northwest of Kimchaek. There were no immediate reports of damage or casualties. China's Earthquake Administration described the tremor as a suspected explosion. <br/><br/>The administration said in a statement on its website that the quake, which occurred around 11.30am local time, was recorded at a depth of zero kilometres. Previous recent tremors in the region have been caused by nuclear tests. The tremor comes hours after North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.