Donald Trump speaks at the National Rifle Association Leadership Conference, Apr. 28, in Atlanta. / AP-Yonhap

U.S. President Donald Trump insisted Friday that South Korea should pay for the U.S. THAAD missile defense system because it's for defending the country, even after Seoul raised objections to the demand.

"Why should we pay for it?" Trump said in an interview with the Washington Times. "It's a phenomenal protective system, best in the world by far, and that's meant to protect South Korea. So I respectfully say that I think it would be appropriate if they paid for it."

Trump first made the THAAD demand in an interview with Reuters on Thursday, despite an agreement reached between the two countries last year that the U.S. military will fund the deployment as long as Seoul agrees to host it and provides land for it.

South Korea said it has no plan to pay for it.

Trump's demand rattled an ally trying to work together with the U.S. 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 U.S. 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.

During the Washington Times interview, Trump also said that he feels the "tremendous weight" of the presidency in grappling with national security challenges such as "the potential of seriously having to counteract North Korea."

"Tremendous pressure's being put on," Trump was quoted as saying. "Now, I don't know that China has the kind of control that some people think. And some people think they have very little control. You know, they've had many wars with Korea. This isn't easy for China."

Trump praised Chinese President Xi Jinping as "a great guy."

"He's a highly respected person, somebody I really, really like, and I believe he's trying," he said. (Yonhap)