North Korea tested a rocket engine earlier this year in a potential bid to further develop its missile capability, a US think-tank said Thursday after reviewing new satellite images.

Images of the Tongchang-ri rocket base in the country's northwest indicated the North conducted at least "one or more rocket engine tests" in late March or early April, the US-Korea Institute at Johns Hopkins University said.

The site -- also known as Sohae Satellite launch station -- was the base for the communist state's successful rocket launch last December, after a failed attempt in April of the same year.

Pyongyang claims both were peaceful science projects aimed at putting its Unha satellite into orbit. The international community, which widely condemned the December launch, views them as disguised long-range ballistic missile tests banned under United Nations resolutions.

Nick Hansen, an expert on imagery analysis, said engine tests were "a key part" of efforts to develop long-range missiles.

He said the latest images showed activities including movements of fuel tanks around a launch pad and a seven-car train potentially carrying the engine, other equipment and technicians to the test site.

They also showed a new distinctive orange-coloured stain in the flame trench -- designed to protect a rocket from exhaust gases -- on the launch pad that is similar to those shown in previous rocket launches, he said.

"Rocket engine tests, while less visible, are also important in technology development," wrote Hansen on the institute's blog, 38 North.

"The recent engine test indicates that Pyongyang continues to move forward with its... long-range missile programmes despite continuing United Nations sanctions and China's public expression of displeasure with the North's efforts to further develop nuclear weapons and the missiles to deliver them," he wrote.

December's rocket launch prompted further tightening of UN sanctions on the impoverished but nuclear-armed state.

Pyongyang responded by staging its third nuclear test in February -- a move that even irritated its sole major ally, China. The isolated North's economic prop made the rare move of joining other UN Security Council members in slapping more sanctions on the regime in March.

Angered by new sanctions and what it called Seoul's hostile policy, the North mounted a series of apocalyptic threats, including of nuclear attacks on Seoul and Washington, sharply raising tension on the peninsula.

In April, Pyongyang withdrew its workers from a joint industrial complex, citing military tensions.

The crisis subsided after Pyongyang changed tack and made a series of reconciliatory gestures towards Seoul and Washington in recent months, though initial talks held over the weekend to discuss reopening the industrial complex were fruitless.