TOKYO (Reuters) - North Korea says it has resumed plutonium production by reprocessing spent fuel rods and has no plans to stop nuclear tests as long as perceived U.S. threats remain, Japan's Kyodo news agency reported on Wednesday.

North Korea's Atomic Energy Institute, which has jurisdiction over the country's Yongbyon nuclear facilities, also told Kyodo it had been producing highly enriched uranium necessary for nuclear arms and power "as scheduled."

"We have reprocessed spent nuclear fuel rods removed from a graphite-moderated reactor," the institute told Kyodo in a written interview.

The institute did not disclose the amount of plutonium or enriched uranium it had produced, Kyodo said, but it has been understood for months that North Korea has resumed plutonium production at the site.

In June, the U.N. nuclear watchdog, the International Atomic Energy Agency, said North Korea appeared to have reopened the Yongbyon plant to produce plutonium from spent fuel, and a senior official of the U.S. State Department said North Korea had restarted production of element.

On Wednesday, the U.S. State Department said it was aware of the reported North Korean comments and called such activities "a clear violation" of U.N. resolutions.

"We call on North Korea to refrain from actions and rhetoric that further raise tensions in the region," said Katina Adams, a spokeswoman for the department.

North Korea vowed in 2013 to restart all nuclear facilities, including the main reactor at Yongbyon, which had been shut down. It said last September that Yongbyon was operating and that it was working to improve the "quality and quantity" of its nuclear weapons.

North Korea conducted its fourth nuclear test in January and tested a long-range rocket the following month, prompting a new round of international sanctions.

Despite the sanctions, a Washington-based research institute said in June, North Korea may be significantly expanding its nuclear weapons production and could have added six or more weapons to its stockpile in the previous 18 months.

Joel Wit, of the Washington-based North Korea monitoring project, 38 North, said the latest North Korean statement was likely to be connected to U.S.-South Korean military exercises due to be held this month.

North Korea regularly denounces such drills as preparations for war.

According to Kyodo, the North Korean institute said it had already succeeded in making "lighter and diversifying" nuclear weapons, and that it had no intention of halting nuclear tests.

"Under conditions that the United States constantly threatens us with nuclear weapons, we will not discontinue nuclear tests," it was quoted as saying.

North Korea will also build a 100,000-kilowatt light-water nuclear reactor for experimental use, the institute was quoted as saying, but it did not provide further details.

(Reporting by Elaine Lies in Tokyo; Additional reporting by David Brunnstrom in Washington; Editing by Robert Birsel and Steve Orlofsky)