The phone call came after Trump said the South should pay for the $1 billion system. The demand sparked a firestorm in South Korea because it runs counter to a deployment deal in which the US agreed to pay for the system in exchange for Seoul's agreement to host it and provide land for it.

"The last thing I would ever do is contradict the president of the United States, you know? And That's not what it was. In fact, what I told our South Korean counterpart is until any renegotiation that the deal is in place. We'll adhere to our word," McMaster said.

"What the president has asked us to do is to look across all of our alliances and to have appropriate burden-sharing, responsibility-sharing. We are looking at that with a great ally, South Korea. We're looking at that with NATO," he said. "And what you've seen because of the president's leadership, more and more nations are contributing more to our collective defense."

Asked if who will pay for THAAD is still up in the air, McMaster said, "The question of what is the relationship on THAAD, on our defense relationship going forward, will be renegotiated as it's going to be with all of our allies."

"Because what the president has said is, he will prioritize American citizens' security and interests. And to do that, we need strong alliances. But also to do that effectively, and a way that is sustainable economically, we need everybody to pay their fair share," he said.

Earlier, the office of South Korean national security advisor Kim released a readout on the phone call with McMaster, but the statement made no mention of "renegotiation," only saying that the two sides reaffirmed their existing agreement.

The office also quoted McMaster as saying during the 35-minute phone call with Kim that Trump's remarks were made in a general context with American people's hopes for defense cost sharing by allies in mind.

Trump first made the THAAD demand in an interview with Reuters on Thursday.

South Korea immediately rejected the demand, but Trump insisted in an interview with the Washington Times the following day that he believes "it would be appropriate if they paid for it."

The demand has rattled an ally trying to work together with the US to tackle the growing nuclear and missile threats from the North. Many South Koreans expressed anger and bewilderment, raising calls for scrapping the deployment altogether.

It was seen as a direct slap in the face to South Korea, which has withstood a series of economic retaliatory measures by China for deciding to host the US system. South Korea has hoped Trump will press Beijing to stop the retaliatory measures, but critics say he instead joined China in bullying a key ally. (Yonhap)