Washington said Tuesday that South Koreans who visited North Korea even once after March 1, 2011 will have to apply for a visa before entering the U.S.

Normally South Koreans can visit the U.S. visa-free and simply fill in an Electronic Travel Authorization form for short-term visits.

But now the U.S. is classifying South Koreans who visited North Korea in the same category as those who traveled to Syria and other Middle East conflict regions.

Between 20,000 and 30,000 South Korean politicians, businesspeople, artists and athletes fall into the category, including Minjoo Party leader Lee Hae-chan, Samsung chief Lee Jae-yong, singers Cho Yong-pil and Baek Ji-young and other entertainers.

The U.S. re-designated North Korea a state sponsor of terrorism on Nov. 20, 2017 following the assassination of Kim Jong-nam, North Korean leader Kim Jong-un's half-brother, and the imprisonment and death of American student Otto Warmbier.

But the sudden visa-rule change has left some South Koreans scratching their heads, including the Foreign Ministry.

One diplomatic source in Washington said, "It looks like the move was prompted by North Korea's reluctance to resume working-level talks with the U.S. as promised during the U.S.-North Korea meeting in Panmunjom in June and recent series of missile provocations."

