South Korea, Japan and the United States Friday were investigating U.S. data showing unusually high ground temperatures at a North Korean nuclear facility, officials said.

South Korean officials said temperatures at the North's Yongbyon plant rose briefly from a normal 50 to 59 degrees to 95 degrees early Tuesday. U.S. officials said that Japanese sensors were monitoring the atmosphere to try to determine whether there had been a ground fire or an operational breakdown.It was not known how long the temperatures remained at elevated levels or whether they have returned to normal.

The Ministry of Science and Technology in South Korea said Friday that American satellite data first showed ground temperatures at the reactor shoot up before dawn Tuesday.

The U.S. officials, speaking on condition of anonymity, said the temperature increase probably was not related to an underground test.