North Korea has restarted its plutonium reactor and expanded its uranium facilities, U.S. Director of National Intelligence James Clapper said Tuesday. The nation could have stockpiles within weeks, he said.

Under the supervision of Supreme Leader Kim Jong Un, Pyongyang has been conducting forbidden missile tests for several years, and U.S. and Chinese authorities said they fear North Korea is looking to create an intercontinental missile. Relations between the U.S. and North Korea have long been antagonistic, and U.S. intelligence officials have made every effort to prevent the nation from acquiring the materials needed to make a nuclear weapon.

"We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor," Clapper said, according to ABC News. "We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor's spent fuel within a matter of weeks to months."

Plutonium is a reactive metallic element that can be used to make nuclear bombs. In order to be usable, however, it needs to be separated from spent fuel — a feat that Clapper said the North Koreans could soon accomplish.

The U.S. dropped a plutonium-type nuclear bomb called Fat Man on Nagasaki, Japan, in 1945 during World War II. The bomb devastated the city, killing 50,000 people. Health professionals suspect that radiation from the bomb has continued to cause health problems such as cancer and birth defects in the city to this day, though the effects are still being studied.

News of North Korea’s reactor restart came less than a week after Pyongyang launched a long-range rocket. North Korean officials claimed the launch was for a satellite, while most world powers, including the U.S., said the launch was a weapons test.