North Korea appears to be preparing for another nuclear test as it conducts a rare ruling party congress, according to recent satellite images published by a US website that monitors the country.
Vehicle activity was monitored at the command centre of the Punggye-ri underground nuclear test site, according to 38 North, a website hosted by the US Johns Hopkins University's school of advanced international studies.
"While the historical record is incomplete, it appears that vehicles are not often seen here except during preparations for a test," 38 North said. "No vehicles or people were observed at this facility on imagery from May 2. However, what appears to be four closely-parked vehicles are present on May 5."
Leaders and members of the Workers' Party of Korea opened the first congress in 36 years Friday, where leader Kim Jong Un praised the country's January hydrogen bomb test as a success.
That test strengthened "the grandeur of the country and its power," Kim said in his opening speech, shown on local television.
The event is the first of its kind since Kim succeeded his late father in power over the isolated country four and a half years ago.
The military "seem to be maintaining a higher state of readiness," an unnamed official of South Korea's Defence Ministry was quoted as saying by Yonhap news agency.
Kim is expected to push through more changes in the party leadership and announce new targets for economic development.
The congress is expected to run for three or four days, according to the Yonhap.
A live-fire military exercise may take place during the congress, and the meeting could see Kim's sister Kim Yo Jong promoted to a minister-level post, Yonhap reported Friday.
Nuclear and missile tests by Pyongyang since the beginning of the year have raised tensions with the US, Japan, South Korea and China, among other world powers.