Seoul quickly dismissed the comments on the Terminal High Altitude Area Defense system, saying that under the Status of Forces Agreement that governs the U.S. military presence in the country, the South would provide the THAAD site and infrastructure while the U.S. would pay to deploy and operate it.

“There is no change to this basic position,” South Korea’s Defense Ministry said in a statement.

The two countries have been in a security alliance since the 1950-53 Korean War, and more than 28,000 U.S. troops are stationed in the South.

Trump said that the THAAD system now being deployed in South Korea to defend against a potential missile attack from North Korea would cost about $1 billion and questioned why the United States was paying for it.

“I informed South Korea it would be appropriate if they paid. It’s a billion-dollar system,” Trump said in a White House interview. “It’s phenomenal, shoots missiles right out of the sky.”

South Korean presidential front-runner Moon Jae-in repeated his earlier position that the current government should stop deploying THAAD and defer the decision to the next president. In a statement, Moon’s camp questioned whether it was in line with the spirit of the security alliance for the U.S. to unilaterally ask South Korea to pay for the THAAD and to scrap the free trade deal.

A top foreign policy adviser to Moon said Trump’s suggestion would be an “impossible option.”

“Even if we purchase THAAD, its main operation would be in the hands of the United States,” said Kim Ki-jung, a foreign policy adviser to Moon and professor at Seoul’s Yonsei University. “So purchasing it would be an impossible option. That was our topic when we were considering the options.”

Moon is leading polls by a wide margin ahead of South Korea’s May 9 election to replace impeached former President Park Geun-hye, whose government agreed with Washington last year to deploy THAAD.

Ahn Cheol-soo, a presidential candidate who supports THAAD, said the U.S. should stick to its agreement and cover the costs.

The U.S. military started the deployment of THAAD in early March, despite strong opposition from China, which says the system’s radar can be used to spy into its territory. The deployment has also prompted a North Korean warning of retaliation.

South Korea said on Wednesday major elements of the system were moved into the planned site in the south of the country and the deployment would be complete for its full operation by the end of this year.

The top U.S. commander in the Pacific, Adm. Harry Harris, said the system would be operational “in coming days.”

South Korea announced in July last year that THAAD would be deployed in the southeastern county of Seonjgu, but residents have protested, citing safety fears over the system’s sophisticated radar and its potential to be a wartime target.

South Korean and U.S. national security advisers agreed on Thursday that the deployment was moving ahead smoothly, South Korea’s presidential office said.

The Pentagon described THAAD last month as a “critical measure” to defend South Koreans and U.S. forces against North Korean missiles.

Asked about Trump’s remarks, a former U.S. State Department official estimated the cost of the system at about $1.2 billion but said the United States would not want to sell THAAD to Seoul.

“We want to retain THAAD in our arsenal, consistent with all other U.S. weapons systems deployed on the Korean peninsula. We own them. We retain them. We have the right to redeploy them,” the official said, speaking on condition of anonymity.

Social media commentators were derisive. “So he wants to start a war with North Korea and he wants South Korea to pay for it,” wrote one Twitter poster.

Elsewhere in the interview, Trump called a 5-year-old trade pact with South Korea “unacceptable” and said it would be targeted for renegotiation after his administration completes a revamp of the North American Free Trade Agreement with Canada and Mexico.

He blamed the trade deal, known as KORUS, on his 2016 Democratic presidential election opponent, Hillary Clinton, who as secretary of state promoted the final version of the pact before its approval by Congress in 2011.

“It is unacceptable, it is a horrible deal made by Hillary,” the Republican Trump said. “It’s a horrible deal, and we are going to renegotiate that deal or terminate it.”

Woo Taehee, South Korea’s vice trade minister, said the country was not notified of any trade renegotiation, and that there have been no working-level talks with the U.S. regarding the five-year-old trade deal.

Woo said the trade ministry was trying to confirm the details of the media reports on Trump’s remarks. He said there have been “no pre-talks” with the U.S. regarding the issue.

KORUS was initially negotiated by the Republican administration of President George W. Bush in 2007, but that version was scrapped and renegotiated by President Barack Obama’s Democratic administration three years later.

The U.S. goods trade deficit with South Korea has more than doubled since KORUS took effect in March 2012, from $13.2 billion in 2011 to $27.7 billion in 2016, according to U.S. Census Bureau data.