Seoul - North Korea defended its right to take "tough counter-measures" in response to what it called US aggression, after firing a ballistic missile over Japan which sparked fear and fury in Tokyo on Tuesday.The test launch by the nuclear-armed nation was seen as a major escalation that triggered global alarm and an angry response from the Japanese government.A visibly unsettled Prime Minister Shinzo Abe said it was an "unprecedented, serious and grave threat", while the UN Security Council called an emergency meeting at Tokyo and Washington's request.But North Korean ambassador Han Tae-Song, addressing the UN Conference on Disarmament in Geneva, said his country had the right to react to ongoing US-South Korean military exercises."Now that the US has openly declared its hostile intention towards DPR (North) Korea by raising joint aggressive military exercises despite repeated warnings... my country has every reason to respond with tough counter-measures as an exercise of its rights to self-defence," Han said, without mentioning the missile launch.Washington, he said, would be responsible for "the catastrophic consequences" that may result from heightened tensions on the Korean peninsula. The North always condemns the annual Ulchi Freedom Guardian exercise and other joint drills as a rehearsal for invasion, while Seoul and Washington say they are purely defensive.Sirens blared out and text messages were fired off across northern Japan Tuesday warning people in the missile's flight path to take cover. Trains were delayed as passengers were urged to seek shelter inside stations."All lines are experiencing disruption," said one sign on Sapporo's metro system. "Reason: Ballistic missile launch."The last time a North Korean rocket overflew Japan was in 2009, when Pyongyang said it was a satellite launch. Washington, Seoul and Tokyo believed it was a clandestine test of an intercontinental ballistic missile (ICBM).In a 40-minute telephone call with US President Donald Trump, Abe said, they had agreed to "further strengthen pressure against North Korea".