The testy reaction -- which came hours after a mammoth nuclear test by North Korea -- alarmed analysts and South Korean government leaders. It came as Trump openly weighs scrapping a US trade deal with South Korea in a bid to fulfill his populist campaign promises. The two factors combined to create the appearance of a rift between the two men at a moment of fresh crisis in the region.
Privately, Trump's aides say the President has grown frustrated by what he regards as a soft stance toward North Korea
by President Moon Jae-in, who has pressed for negotiations with Pyongyang in an attempt to tamp down rapidly heightening tensions.
The disagreement burst into the open on Sunday when Trump tweeted a broadside against the US ally.
"South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!" he wrote. He didn't specify what the "one thing" was, but has suggested in the past that shows of military force are necessary to halt the country's progress toward nuclear armament.
The unusual criticism of a top US partner in the region raised concern, including within Moon's government, which requested a formal explanation from the White House.
In a letter, the US National Security Council wrote there is "no difference of view between South Korea and the United States," according to a ranking official in Moon's government quoted by the state-run Yonhap news agency. The White House declined to comment on the letter.
In their phone call, Trump and Moon agreed to lift restrictions on the payload weight of South Korea's ballistic missiles, according to a South Korean presidential spokesman. South Korea was previously restricted to payload weights of up to 1,100 pounds under a 2012 agreement.
The official reassurances, however, have done little to dampen the view that Trump and Moon are on the outs. Though most analysts and diplomats have grown accustomed to Trump's combative manner, even with his supposed allies, the recent bombast has done little to comfort an anxious region.
"The first thing we need to do is to cool down the rhetoric against our main ally in the region, South Korea," said Bill Richardson, the former New Mexico governor and US ambassador to the United Nations. "We shouldn't threaten abolishing the free trade agreement, the appeasement talk. This is the way the South Korean president was elected. He wanted a dialogue with North Korea, try to stamp out corruption, so this is his own internal politics. I think we have to stand behind South Korea."
Moon took office in May, replacing impeached President Park Geun-hye. Since then, he's found himself wedged between increasingly bellicose rhetoric from both Washington and Pyongyang.
In August, he issued a forceful statement rebuking Trump's threats of "fire and fury" against North Korea, saying that any military actions on the Korean peninsula must be made in consultation and with agreement from Seoul. He's raised the notion of military talks between North and South as a way to possibly ease the growing strain.
While Trump has repeatedly said that "all options" are being considered for North Korea, he seemed to rule out talks in a tweet last week.
"Talking is not the answer," he wrote. Later, top US military and diplomatic officials insisted that negotiations had not been taken off the table in dealing with North Korea.
Moon last spoke with Trump on Friday, but their conversation centered on the trade issue, according to a senior administration official. Trump told reporters over the weekend that he was discussing the issue of trade with South Korea with his advisers.
"It's very much on my mind," he said as he traveled in Texas. Aides say Trump is irritated that Moon hasn't accepted certain US demands in attempts to renegotiate the trade deal, which was reached in 2007, and has asked advisers to begin plans to terminate the agreement.
Over the weekend, Trump spoke with Japanese Prime Minister Shinzo Abe twice, once before the nuclear test and once after. The White House declined to say why Trump phoned Abe before calling Moon, a sequence that raised eyebrows in Seoul.
The distance between the two sides -- or even the appearance of a dispute -- raised concerns as North Korea issues escalating threats.
"I'm sure that Pyongyang enjoys seeing us fight with our own ally in the region," said Rep. Adam Schiff, the leading Democrat on the House intelligence committee, on Sunday's "CNN State of the Union."
"I'm not sure what the President's point is, particularly today, at a time when South Korea is feeling very threatened, as indeed we are to be lashing out at South Korea and saying that they're making a mistake or they're doing things wrong," Schiff said.