SEOUL, South Korea — South Korea’s main intelligence agency confirmed on Tuesday that North Korea has restarted a Soviet-era nuclear reactor that has been used to obtain plutonium for bombs, according to South Korean legislators.

Nuclear experts in the United States, including at the U.S.-Korea Institute at Johns Hopkins University, reported last month that satellite photographs indicated that North Korea had restarted the five-megawatt reactor at its main nuclear complex in Yongbyon, north of Pyongyang, the capital.

Two lawmakers — Cho Won-jin from the governing party and Jung Cheong-rae from the opposition — told reporters at a joint news briefing on Tuesday that Nam Jae-joon, the director of the National Intelligence Service of South Korea, confirmed the restart during a closed parliamentary session. The two lawmakers were designated by their parties to relay the news to reporters.

A spokesman for the intelligence service refused to confirm or deny the legislators’ comments.

North Korea has yet to confirm the reported resumption of nuclear activity, although it had repeatedly warned that it was preparing to restart it after a six-year hiatus.