South Korea could let American troops leave the country if one of the liberal candidates is elected president next year and incoming US President Donald Trump demands Seoul pay more for the troops, a US expert said Wednesday.

Max Boot, a senior fellow at the Council on Foreign Relations, issued the warning in an article in Foreign Policy magazine, saying that troop withdrawal from South Korea could significantly weaken America's standing in the Asia-Pacific region.

The expert also noted that Trump has "long-standing hostility to American allies, whom he sees as ungrateful freeloaders, and to free trade, which he sees as a rip-off," even though he assured South Korea and Japan after the election that he won't abandon US alliances.

"But what will happen if he demands, as he has said he will do, that South Korea and Japan pay more for the privilege of being protected by the United States? Prime Minister Shinzo Abe and President Park Geun-hye are pro-American enough that they might agree," Boot said.

However, Park is now in the process of being ousted in impeachment proceedings, the expert said, adding that the leading opposition candidates to succeed her include what he called the "leftist Moon Jae-in and the populist Lee Jae-myung."

Moon and Lee "are considerably less pro-American," he said.

"If one of them wins the presidency, and Trump demands more payment for protection, South Korea just might let US troops leave without a fight. And if that were to happen, America’s standing in the Pacific region would plummet," the expert said.

About 28,500 American troops are stationed in South Korea to deter North Korean aggression, a legacy of the 1950-53 Korean War, which ended in a truce, not a peace treaty. Many agree that the troop presence is also in line with US interests in a region marked by China's rise.

South Korea currently shoulders about half the cost of keeping the American troops in the country. But Trump argued during the election campaign that Seoul should pay up to even 100 percent of the cost or the US should be prepared to leave the country.

Since his election, however, Trump has shown signs of retreat from his campaign rhetoric, making remarks reaffirming the alliance with South Korea during a phone call with President Park Geun-hye a day after election, and filling some key posts with people valuing the alliance. (Yonhap)