North Korea may be preparing for a fifth nuclear test, judging by satellite images taken on May 5, a US think-tank says.
The 38 North website, run by Johns Hopkins University's School of Advanced International Studies, said on Saturday (local time) the images showed vehicle movement at the country's known test site "not often seen there except during preparations".
There has been intense speculation communist North Korea may conduct its fifth test of a nuclear device during a congress of the ruling Workers' Party now under way.
The analysis from 38 North said the imagery of the Punggye-ri test site "suggests Pyongyang may be preparing for a nuclear test in the near future".
It said while the images continued to show low levels of activity, the vehicles had been spotted at what was believed to be the Command Centre, 6km south of the main site.
A senior US official asked about the 38 North report repeated a call for North Korea "to refrain from actions and rhetoric that further destabilise the region" and stick to past commitments to denuclearise.
"Our commitment to the defence of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad," he added. "We are prepared to defend ourselves and our allies from any attack or provocation."
The 38 North website reported last month that satellite images showed North Korea may have resumed tunnelling at Punggye-ri, activity similar to that seen before the country's most recent nuclear test in January.
The website reported in early December that satellite photographs from the two previous months indicated North Korea was digging a new tunnel for nuclear testing.
North Korea has vowed to conduct more nuclear testing in spite of stepped-up international sanctions after its previous test and a subsequent space rocket launch seen as a cover for development of its intercontinental ballistic missile program.