Gallucci said the North has a record of building a plutonium production reactor in Syria, though the facility was ultimately destroyed by an Israeli strike in 2007 before it became operational.

"It is this type of activity, selling fissile material, the equipment or technology to produce it, nuclear weapons components or designs, or even the weapons themselves, that would create the nightmare scenario of nuclear terrorism we most fear," Gallucci said.

"Taking an early opportunity to underline for Pyongyang that such transfers will be met with a swift retaliatory response would be a good idea," he said.

Gallucci said there are three options to deal with the North -- containment, military force and negotiation -- but none of them appear effective alone. He suggested mixing the options, saying those options shouldn't be regarded as mutually exclusive.

"Perhaps a strategy built from each of them has some chance of success," he said.

Should the administration of President Donald Trump be willing to use the option of negotiations, it shouldn't focus on sanctions, Gallucci said.

"We for example don't particularly want to go to the negotiating table on the heels of a nuclear test or on the heels of a long-range ballistic missile test because it appears both domestically and internationally as though we're being pressured to the table," he said.

"That's not the way a negotiator likes to go to the table. Not surprisingly, the North Koreans have a similar view and they would like for us not to introduce our effort at engagement by first starting with sanctions," he added. (Yonhap)