The 38 North website, run by Johns Hopkins University, reported that commercial satellite images taken Thursday showed the presence of vehicles at a North Korean nuclear command center, which could be a sign that a nuclear test was imminent.

"While historical record is incomplete, it appears that vehicles are not often seen there except during preparations for a test," analysts at the US-Korea Institute at Johns Hopkins University said.

The center of activity lies 4 miles (6.5 kilometers) from the test site at Punggye-ri. The website said the level of activity at the site continues to be low.

Responding to the 38 North report, a senior US official called for North Korea "to refrain from actions and rhetoric that further destabilizes the region."

"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad," the official said on condition of anonymity. "We are prepared to defend ourselves and our allies from any attack or provocation," he added.

'Great success'

On Friday, North Korean leader Kim Jong Un used the ruling Workers' Party's congress to defend his country's nuclear program.

The congress takes place at a time of rising tensions on the Korean peninsula, following the North's alleged hydrogen bomb test in January and the launch of a long-range ballistic missile on February 7.

Kim described a hydrogen bomb test in January and the launch of an Earth observation satellite earlier this year as a "great success."

At the first Workers' Party congress in 36 years, Kim said he would not be cowed by sanctions and international pressure.

shs/bw (Reuters, AP)