Washington this year severed food assistance for North Korea after Pyongyang reneged on a pledge to halt missile and nuclear tests with a failed April attempt to send a long-range rocket into orbit. Similar launches in 2006 and 2009 coincided with North Korean nuclear tests.
The U.S.-Korea Institute at Johns Hopkins University said information gleaned from commercial satellite imagery suggests North Korea has tested at least two rocket motors as recently as September.
"Despite its failed rocket test last spring, commercial satellite imagery indicates that North Korea continues to develop long-range missiles, possibly with intercontinental ranges," a statement posted on the institute's 38 North blog reads.
In August, the Bulletin of the Atomic Scientists stated the completion of a tunnel near the site of North Korea's first nuclear tests suggested Pyongyang might be preparing for another test.
Analysts at IHS Jane's Defense Week, meanwhile, had told The New York Times that North Korea made good progress on the construction of a light-water uranium reactor this year.
Enriched uranium could find its way into the North Korea nuclear arsenal. The country's first nuclear tests were believed to have used plutonium devices.
The state-run Korean Central News Agency in October said U.S. interests are within reach of North Korean missile systems.
Tensions on the Korean Peninsula escalated last year following the death of North Korean leader Kim Jong Il.