It said the DPRK is believed to have between 2,500 and 5,000 tons of chemical weapons, including mustard gas, phosgene, blood agents and sarin. These weapons can be delivered with ballistic missiles and long-range artillery and are "sufficient to inflict massive civilian casualties on South Korea (ROK)."

"If progress is made on rolling back Pyongyang's nuclear ambitions, there could be opportunities to construct a cooperative diplomatic solution for chemical weapons and the suspected biological weapons program," the think tank said in a report released Wednesday night.

It also called on the US to engage Pyongyang in dialogue to defuse the nuclear crisis, saying "diplomacy is the least bad option." It said sanctions won't resolve the problem on their own, and military force is not an option.

In a rare move, leaders of Russia and China used their meetings in Moscow on Wednesday to pressure the DPRK to return to the nuclear talks and expressed "serious concerns" about tension on the Korean peninsula.

"Russia and China are ready to foster the lowering of tension in Northeast Asia and call for the continuation of efforts by all sides to resolve disagreements through peaceful means, through dialogue and consultations," their statement said.