SEOUL: Whatever the nature of the device, it was North Korea’s fourth nuclear test and marked a striking act of defiance in the face of warnings from enemies and allies alike that Pyongyang would pay a steep price for moving forward with its nuclear weapons programme.

The North’s official news agency was unrepentant.

US “imperialists” had escalated the situation on the Korean peninsula to the brink of war, defying the North’s calls for a peace treaty, it said.

“The more frantic the hostile forces get in their moves to isolate and stifle the DPRK (North Korea), the stronger its nuclear deterrent will grow, bringing them to deathbed repentance.” The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions.

Their failure to prevent a fourth detonation will put the Security Council under intense pressure to take more drastic action this time around.

It throws down a particular challenge to US President Barack Obama, who, during a visit to South Korea in 2014, vowed sanctions with “more bite” if Pyongyang went ahead with another test.

The final response of China will be key. Beijing has restrained US-led allies from stronger action against Pyongyang in the past.