Moon Jae-in, the presidential candidate of the Democratic Party of Korea, attends a televised presidential debate in Seoul on April 28. (Seongjoon Cho/Bloomberg News)

The South Korean government reeled Friday over President Trump’s sudden insistence that he expects Seoul to pay $1 billion for a missile defense system that many here do not want, the latest in a series of slights against one of the United States’ leading allies in Asia.

Trump’s remarks come at a particularly sensitive time on the Korean Peninsula: Not only have tensions with North Korea risen to their highest level in years, but South Koreans are heading to the polls next month and could elect a president whose ideas about how to deal with North Korea are very different from Trump’s. 

“So far the reaction in South Korea to all these things that Mr. Trump has said has been surprisingly restrained, but I think that’s because South Koreans are still trying to figure out what kind of character he is,” said David Straub, a former U.S. diplomat dealing with the Koreas and author of the book “Anti-Americanism in Democratizing South Korea.”

“They know he’s an unusual president and they’re discounting a lot of what he says, but eventually remarks like these will have a serious effect,” Straub said.

(Jason Aldag/The Washington Post)

Trump on Thursday revived — in a particularly blunt way — his campaign-trail complaints that South Korea was not paying enough for its defense and was getting the better of a trade deal. 

This followed his assertion that Korea was once part of China — which angered many South Koreans — and his phone calls to Beijing and Tokyo over the North Korean problem but not to Seoul. 

South Korea is between presidents after the impeachment of Park Geun-hye last month, but many people here took offense that Trump did not call their acting president on an issue involving the Korean Peninsula. 

In the latest twist, Trump said he wants the government in Seoul to pay for the Terminal High Altitude Air Defense (THAAD) system that the U.S. military installed in South Korea this week and that is expected to become operational in the next few days.

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

The South Korean government had been reluctant to deploy the system, aimed at guarding against the North Korean threat, because it would anger China, its biggest trading partner.  

South Koreans demonstrate on April 28 against the deployment of an advanced U.S. missile defense system called Terminal High-Altitude Area Defense, or THAAD, near the U.S. Embassy in Seoul. (Lee Jin-man/AP)

The U.S. military urged Seoul to deploy the system, and the government agreed to it in July. Under the accord, the United States would pay for the system while South Korea would supply the land for it. 

But it has remained controversial. Fierce opposition mounted when the U.S. military moved key pieces of THAAD equipment onto the deployment site in the middle of the night this week. 

Pressure to make South Korea pay for THAAD is likely to boost support for liberal presidential candidate Moon Jae-in, who has vowed to review the previous government’s decision to host the system.

Moon already has a strong lead in polls ahead of the May 9 election. Among his policy proposals, he wants to resume engagement with North Korea, pursuing a starkly different course from Trump’s “maximum pressure” approach.

When progressives were elected president before — Kim Dae-jung in 1997 and Roh Moo-hyun in 2002 — anti-American sentiment rose sharply on both occasions, Straub said.

Moon’s team seized on Trump’s statement.

“It has become obvious that the deployment decision process was seriously flawed from the start,” Youn Kwan-suk, a spokesman for the Moon campaign, told reporters in Seoul. 

“As the THAAD system will have a big impact on our security and impose huge economic costs, it’s essential that we seek approval for it from the National Assembly,” Youn said, reiterating Moon’s intention to review the plan.  

Centrist Ahn Cheol-soo, who has pledged to abide by the previous government’s deal with the United States, said that the agreement must be honored and that any changes would need National Assembly approval. 

Even conservative parties that support the THAAD deployment said the issue was “a matter of trust” between the allies. 

In another slight, Trump said the U.S. trade agreement with South Korea — forged during the George W. Bush administration and implemented in 2012 — was “a horrible deal” and threatened to terminate it

“It was a Hillary Clinton disaster, a deal that should’ve never been made,” he said in an interview Thursday night with The Washington Post. 

During a visit to South Korea earlier this month, Vice President Pence said the administration wants to overhaul the deal to make it fairer to the United States.

South Korean authorities said Friday that they had not been informed of either proposal.

“Our basic position remains unchanged,” South Korea's Defense Ministry said in a statement. The THAAD agreement reached last year remains in force, it said. 

A Trade Ministry official said Seoul has not received a request to renegotiate the trade deal or a notice scrapping it. “We are trying to understand what President Trump said exactly and the context of his remark,” a ministry official told local media.