John Hopkins researchers, writing on their 38 North blog Thursday, said commercial satellite imagery of the facility suggests North Korea has "probably" restarted the plutonium production reactor.
A satellite image posted on the site shows plumes of white smoke coming from a building labeled as a turbine and generator facility.
"The white coloration and volume are consistent with steam being vented because the electrical generating system is about to come online, indicating that the reactor is in or nearing operation," the researchers said.
North Korea sparked international concern in April when it announced it would resume activity at the Yongbyon nuclear facility. Johns Hopkins said it expected the facility would be restarted by the end of August.
North Korea conducted an underground test of a nuclear device in February, its third. Johns Hopkins researchers said the Yongbyon reactor can produce 6 kilograms of plutonium a year, which North Korea can use "to slowly increase the size of its nuclear weapons stockpile."
The Yongbyon nuclear complex was closed in 2007 following multilateral negotiations. Talks between both Koreas, China, the United States, Russia and Japan were suspended in 2009.