Japan, South Korea and the U.S. should swiftly map out a game plan aimed at freezing North Korea's nuclear activities, a senior researcher at a British military think tank said Tuesday in Tokyo.

"The immediate objective is to achieve a halt of North Korea's nuclear activities so that North Korea does not benefit from a long and difficult negotiation," said Gary Samore, director of studies at the International Institute for Strategic Studies.

Samore was one of the U.S. negotiators who hammered out the 1994 Agreed Framework, which put Pyongyang's nuclear program on ice.

He said that diplomatic efforts being carried out under the six-way framework -- comprising Japan, the U.S., South Korea, China, Russia and North Korea -- should also focus on setting up a concrete deadline for reaching an agreement on measures that would eventually lead to the dismantling of North Korea's nuclear capabilities.

Meanwhile, Adam Ward, a senior fellow for East Asian security at the institute, noted that the key point is not only to freeze North Korea's plutonium reprocessing plants but also its program for highly enriched uranium.

Washington apparently fears that some nations within the six-party framework may be tempted by the prospect of a freeze on the plutonium facilities while ignoring the enrichment program.

According to an IISS report earlier this month, North Korea could have four to eight nuclear weapons within the year, with the ability to produce about one every year.

In a worst-case scenario, Pyongyang could boost its nuclear production capacity from one bomb a year to eight to 13 if it finishes building a new reactor and a uranium enrichment plant, the report says.