North Korea appears to have blown up tunnels at its only nuclear test site, in a move to reduce regional tensions.
Foreign journalists at the Punggye-ri nuclear testing site in the north-east said they witnessed a huge explosion.
Pyongyang offered to scrap the site earlier this year as part of a diplomatic rapprochement with South Korea and the US.
But scientists believe it partially collapsed after the last test in September 2017, rendering it unusable.
Independent inspectors were not allowed to witness the process of the dismantling of the Punggye-ri site in the mountainous region of the country, and some worry it could be easily reversible, the BBC’s Laura Bicker reports.
This comes ahead of a planned summit between US President Donald Trump and North Korean leader Kim Jong-un in Singapore on 12 June.
However, in recent days both countries have said the meeting could be delayed or even called off.
What happened on Thursday?
Three tunnels were collapsed in a series of explosions in front of about 20 handpicked international journalists.
Two blasts were reportedly carried out in the morning, and four in the afternoon.
Sky News’ Tom Cheshire was among the journalists present. He said the doors to the tunnels were “theatrically rigged” with “wires everywhere”.
“We hiked up into the mountains and watched the detonation from about 500m away (550 yards),” he was quoted by Sky News as saying.
“They counted it down – three, two, one. There was a huge explosion, you could feel it. Dust came at you, the heat came at you. It was extremely loud.”
Test devices are buried deep at the end of the tunnels, which end in a hook.
The tunnel gets backfilled to prevent radioactive leakage and then the device is detonated.
What would it take for North Korea to truly denuclearise?
Pyongyang’s reported dismantling of the site is seen by analysts as a welcome first step.
But it could indicate that it believes its nuclear programme has made sufficient progress and full testing is no longer needed, Catherine Dill from the Middlebury Institute of International Studies (MIIS) says.
The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) could have confirmed that the test site is no longer capable of conducting nuclear tests – but experts from the UN-backed monitoring group were not invited to Thursday’s dismantling of Punggye-ri.
Satellite imagery will be used by governments and independent experts to monitor for activity, new buildings and equipment, which might indicate that North Korea plans to resume testing.
Satellite imagery may not help if North Korea clandestinely opens a new nuclear test site, Catherine Dill says. It has many other mountains that could be used.
But if that were the case, it would be unable to hide any new underground tests, as the resulting seismic tremors would be detected.
She says North Korea’s nuclear weapons programme also goes far beyond the existence of one site.