Sen. Ben Cardin (D-MD), ranking member of the Senate Foreign Relations Committee, also urged Trump not to "divide" the alliance with the South at a time when tensions with North Korea are escalating.

"South Korea is our friend and ally. @realDonaldTrump should not divide our alliance, esp. with ongoing NK crisis," Cardin said in a tweet.

Sen. Ed Markey (D-MA) also issued a statement criticizing Trump's "disrespectful" rhetoric against Seoul.

"The president's talk of war and inconsistent messages from administration officials can only fuel Kim Jong-un's paranoia and increases the chances of an accidental war that could kill millions of people on the Korean Peninsula," the senator said.

"And instead of the president's damaging and disrespectful rhetoric towards South Korea, we need to reaffirm and strengthen our alliance with this key partner that is home to millions of people, including American troops and their families," he said.

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 US military will fund the deployment as long as Seoul agrees to host it and provides land for it.

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. (Yonhap)