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 Koreans` preparations on Tuesday were interrupted by the incident, which prompted fears of spying, with reports stating that the drone was brought down by Mexico`s military.

"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.

Mexican federal authorities did not immediately respond to a request for comment, while officials in the state of Jalisco told AFP they had no formal record of the incident.

Drone flights have been banned over stadiums and facilities hosting World Cup matches and events in Mexico, as well as the United States.

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.