North Korea has made progress in recent months on a home-built reactor that experts fear could be used to bolster the communist state's nuclear weapons program, satellite images showed.

The images, released this week by a private Washington-based think-tank, were taken on February 3, nearly a month before February's surprise nuclear deal between North Korea and the United States.

The images of the Yongbyon nuclear site showed that the new light-water reactor's outside building appeared to be complete. Earlier pictures taken in September showed that it was still under construction.

However, the dome remained on the ground, indicating that the reactor was not yet operational.

"The most significant part is that construction is continuing," said Paul Brannan, a senior research analyst at the Institute for Science and International Security, which released the images.

"In North Korea, projects can sometimes go in fits and starts, so to see any progress tells you something," he said Tuesday.

North Korea first disclosed construction on a new reactor in 2010 to visiting US scientists and also showed them a uranium enrichment plant that was said to produce low-enriched fuel for the new facility.

While both were ostensibly for civilian power, the enrichment plant could be converted to produce highly enriched uranium for bombs. Scientists say that the light water reactor could also be run to produce plutonium.

North Korea, whose nuclear program is based on plutonium, has tested two nuclear bombs.

Under the February 29 agreement, North Korea agreed to a new halt in uranium enrichment along with nuclear and missile tests and to allow back UN inspectors. The United States said it would give North Korea some 240,000 tonnes of badly needed food aid.

North Korea said that the United States agreed to consider providing light-water reactors, a step Washington says would only come as part of new full-fledged six-nation talks on permanently ending the North's nuclear program.