A top Senate Democrat is worried that South Korea, a major American ally in Southeast Asia, is increasingly looking to "partner" with the Chinese government on their most critical national security concern — North Korea.

That concern is the result of a decision by South Korean President Moon Jae-in to suspend the deployment of a U.S. missile defense system that could protect against the danger of incoming North Korean rockets. China has long protested the deployment of the system over fears that the radar could be used against their military.

"It's my fear that he thinks — I hope I'm wrong — that [Moon] thinks that South Korea has a better chance working with China to contain North Korea than working with the United States," Senate Minority Whip Dick Durbin, D-Ill., told the Washington Examiner.

The United States and South Korea have been allies for decades and fought together in the Korean Conflict against North Korea. China backed North Korea in that conflict, which is technically ongoing because there was only an armistice rather than a peace treaty. Durbin and other American officials have tried to persuade Moon to allow the deployment of the weaponry, called a Terminal High Altitude Area Defense system, particularly given the thousands of U.S. military personnel deployed to the country.

"I said to him [last week] if I were living in South Korea I would want missile defense in South Korea and I don't understand why you don't," Durbin told the Washington Examiner, describing a conversation with the South Korean president. "What he told me is they wanted to go through due process, he thought their assembly would approve it. I can't understand the delay, why they even need to vote on it, but I said, ‘proceed with this, but it's $900 million-plus from the United States we're spending to put this in place and then to maintain it. So, I mean, from where I'm sitting, it's a pretty good deal for the South Koreans."

China has used economic measures to retaliate against South Korea for allowing the THAAD to go through, thereby undermining support for the missile defense system in the South Korean business community.

"I think he is trying to find a diplomatic way to slow down the process to placate the business community and placate his political supporters," Stephen R. Nagy, senior associate professor of politics and international studies at International Christian University in Tokyo, told the New York Times.

The United States can hardly require the South Koreans to accept protection, as Durbin acknowledged. "We are protecting them and protecting ourselves because the new Camp Humphries south of Seoul — which they paid for, the South Koreans paid for 92 percent of it — but that Camp Humphries is a huge military facility and I believe that the THAAD system is part of the defense of that facility," he said.