North Korea could have already started reprocessing spent nuclear fuel to harvest weapons-grade plutonium in a process that could give Pyongyang enough fissile material for up to three nuclear bombs, a U.S. research institute said Tuesday.

The Institute for Science and International Security, a think tank in Washington that specializes in the Iranian and North Korean nuclear issues, raised the possibility, citing an unidentified "government official who monitors the situation closely."

The report came a day after 38 North, a U.S. website focusing on North Korea issues, also raised the possibility of the North reprocessing spent fuel, saying recent commercial satellite imagery showed "exhaust plumes" from a thermal plant used to heat the reprocessing facility.

These assessments are in line with a statement that Director of National Intelligence James Clapper made last month that the North had restarted its main five-megawatt nuclear reactor at Yongbyon and has since run it for long enough to harvest plutonium "within a matter of weeks to months."

"If North Korea has indeed started, or even partially completed, this plutonium separation process, the question becomes: how much plutonium for nuclear weapons could it separate? The amount of plutonium separated will depend on the amount of plutonium produced in the 5-megawatt reactor since it restarted in mid-2013," ISIS said.