North Korea has earned worldwide condemnation after exploding a new nuclear weapon, and test-firing what the South says were three short-range missiles. Coupled with the recent launch of a long-range rocket with the potential of carrying a bomb, today’s tests put Pyongyang’s critics on full alert. President Obama said he was gravely concerned, and South Korea said it threatens the safety of the world.“We’re seriously concerned about the North Korean second test of a nuclear device,” said the South Korean Foreign Minister Yu Myung-hwan. “It’s a direct threat against peace and stability in the region and in the rest of the world.” Sweden’s Foreign Minister echoed those concerns and said North Korea is being deliberately difficult. “It’s alarming, it’s a continuation of the provocative behaviour of the regime in Pyongyang,” said Carl Bildt. “I think it further isolates the country, I think it further aggravates the long-term situation of the country, and, of course, it’s not good for the international atmosphere and on the Korean peninsula. So, it is a most disturbing and alarming development.” The underground explosion was detected in the far north-east of the reclusive country, near the Chinese border. Russian scientists measured it at 20 kilotonnes. North Korea argues it has no choice but to defend itself in a hostile world, and threatened more such tests unless the United States stops what Pyongyang called its “campaign of intimidation.” The United Nations Security Council will meet in emergency session later today.