US President Donald Trump launched criticism at South Korea on Monday, just hours before a summit with its new president, Lee Jae Myung, as conflict over defence spending and trade tests the two countries' decades-old alliance.

The leaders were gearing up for their first talks when Trump said on social media, without evidence, that there "Seems like a Purge or Revolution" in South Korea and that "We can't have that and do business there."

The remarks cast a dark mood over what, for Lee, are high-stakes talks.

He took office in June following a snap election called after his conservative predecessor, Yoon Suk Yeol, feted in Washington for his hardline on North Korea, was removed for his December attempt to impose martial law.

South Korea's economy relies heavily on the US, with Washington underwriting its security with troops and nuclear deterrence. Lee hopes to chart a balanced path of cooperation with the US, while not antagonising top trade partner China.

The source of Trump's complaint was not immediately clear. But, for months, supporters of former President Yoon have hoped Trump would intercede in what they say is communist persecution of the impeached ex-president.

Yoon is now on trial on charges of inciting an insurrection. His wife is facing possible indictment on corruption and bribery cases. (Reuters)