If Washington can provide such reassurances to Seoul, "that gives us more scope to really go after the North Koreans," Hill said.

Trump has often linked security cooperation with trade issues. He has pressured South Korea to address its trade surplus with the US and shoulder a larger burden of the cost of stationing American troops there.

Hill called for "much greater emphasis" on the durability of the alliance and less on the transactional aspects.

The former ambassador to Seoul also urged the administration to work harder to bring China on board to denuclearize the North.

"This is not about tweeting to them. This is not about having some jetlagged Secretary of State arrive for an 18-hour visit," Hill said, referring to Trump's tweets and last year's visit to Beijing by Secretary of State Rex Tillerson. "We are not going to be able to pull a rabbit out of the hat with China unless we've spent time stuffing that rabbit down the hat. I think we need some serious diplomacy on this."

Hill took a swipe at the administration for being slow to name key diplomats to handle the North Korea issue.

"In order to have diplomacy, you usually need diplomats," he said. "The fact that we don't have an ambassador (in Seoul) -- we're in the 14th month of that -- the fact that we don't have a negotiator or someone keeping track of where the negotiations might be, I think is also problematic."

The top US diplomat for North Korea policy, Joseph Yun, announced his retirement this week at a time when the two sides have been exploring avenues for talks. (Yonhap)