WASHINGTON -- North Korea has likely produced an undetermined amount of additional plutonium for nuclear weapons since September, an analysis of thermal imagery showed Friday.

38 North, a US website specializing in North Korea analysis, said in a report that the communist country appears to have increased its plutonium production at the main nuclear complex in Yongbyon from September through June.

"The Radiochemical Laboratory operated intermittently and there have apparently been at least two unreported reprocessing campaigns to produce an undetermined amount of plutonium that can further increase North Korea's nuclear weapons stockpile," the report said.

The finding indicates there was batch, not continuous, processing of spent fuel rods in the period of analysis, it said.

North Korea has conducted five underground nuclear tests since 2006, and carried out its first test of an intercontinental ballistic missile last week. Experts believe it is only a matter of time before the North develops an ICBM capable of carrying a nuclear warhead to the US mainland.

The report also noted increased thermal activity at the uranium enrichment facility in Yongbyon.

"It is unclear if this was the result of centrifuge operations or maintenance operations," it said. "Centrifuge operations would increase the North's enriched uranium inventory; however, based on imagery alone, it is not possible to conclude whether the plant is producing low or highly enriched uranium."

Pyongyang is thought to have used plutonium for its nuclear tests in 2006 and 2009, but some analysts say it could have used highly enriched uranium for the fifth test in September.

If confirmed, the development would mark an advance in the North's nuclear weapons program because uranium enrichment is harder to detect than plutonium enrichment. Uranium can also be enriched continually while plutonium stocks are limited. (Yonhap)