North Korea says it’s successfully carried out its fifth nuclear test.
The statement on state media came hours after a seismic event was discovered near its nuclear test site.
South Korea believes it’s the North’s biggest-ever evaluation, raising fears critical atomic progress have been made by it.
The US warned of “serious consequences”.
China’s foreign ministry urged North Korea to avoid additional actions that would worsen the situation and said Beijing was opposed to the evaluation.
The North said the test had been of a “newly developed nuclear warhead” and that it was now capable of mounting a nuclear device on ballistic rockets.
Approximations of the explosive output of the latest blast have changed. South Korea’s military said it was about 10 kilotonnes, enough to make it the North’s “most powerful nuclear test ever”. Other experts say first indications imply 20 kilotonnes or more.
The bomb had a yield of about 15 kilotonnes.
Ms Park, who’s cutting short an overseas visit, said the test was a “dangerous challenge” to the international community that would “just earn more sanctions and isolation” for North Korea.
“Such provocation will further accelerate its path to self-destruction,” she said.
Japan’s Prime Minister Shinzo Abe said his country “certainly cannot condone” any such test and would “protest adamantly” to Pyongyang.
“North Korea’s nuclear development has become a more serious danger to Japan’s security and severely undermines the peace and safety of the area and the international community,” he said.
The White House said President Barack Obama had spoken to both Mr Abe and Ms Park after the latest test.
A statement from press secretary Josh Earnest said Mr Obama had “reiterated the unbreakable US commitment to the security of our allies in Asia and around the world”.