Coach Hong Myung-bo on Wednesday said a drone was spotted flying over South Korea's training session before their eagerly awaited clash with World Cup co-hosts Mexico, calling the incident "unfortunate".
Victory in South Korea's match against Mexico in Guadalajara on Thursday would virtually guarantee progress to the knockout rounds, and put the winners in pole position to top Group A, which also contains South Africa and Czechia.
But the Taegeuk Warriors' preparations on Tuesday were interrupted by the incident, which prompted fears of spying, with Korean news agency Yonhap reporting that the drone was brought down by Mexico's military using radio signals.
"Yesterday during our training there was a drone in the sky, and we came to know about the fact. But fortunately it was right before we practiced our tactics, so it did not impact us significantly," said the coach.
"But, while we were preparing for the match, that was the most important timing. So, what happened was unfortunate," he told a press conference.
Drone flights have been banned over stadiums and facilities hosting World Cup matches and events in Mexico, as well as the United States.
Yonhap said a South Korea team security officer spotted the device, and a Mexican military drone-interdiction specialist stationed at the training camp brought it down by emitting radio signals.
Two men who were suspected to be the drone operators retrieved the crashed device and fled the scene.
The Korea Football Association has filed a report to FIFA and requested reassurances that there will not be a repeat.
Mexican federal authorities did not respond to an AFP request for comment, while officials in the state of Jalisco said they had no formal record of the incident.
Meanwhile, Korean coach Hong said he expected a challenging clash against Mexico, praising the opponents' "creative" midfield and telling reporters he expected a vocal home crowd to play a role.
"We fully understand that it's going to be a match with the home team, and we know that that's going to give benefits to the home team," he said.
"But my players have experienced such matches before, so it will be different tomorrow, and we need to control the rhythm and the flow of the match."
Mexico beat South Africa 2-0 in the tournament's opening game last week, while South Korea came from behind to defeat Czechia 2-1.
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