Story highlights Trump's aides say the President has grown frustrated with South Korea

The unusual criticism of a top US partner in the region has raised concern

Washington (CNN) President Donald Trump spoke by phone Monday with his counterpart in South Korea, their first conversation since Trump directed harsh criticism at the new liberal leader's push for negotiations with the North.

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! — Donald J. Trump (@realDonaldTrump) September 3, 2017

"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.

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