The gesture has been interpreted in both Japan and Korea as an intentional mockery of the Japanese. Apparently Koreans sometimes insult the Japanese by referring to them as monkeys [倭猿?].
After Korea lost the match, Ki was interviewed about the monkey performance. He claimed it had no particular meaning. However, messages later posted on his Twitter account stated that he had been angered by the sight of rising sun flags in the stadium. He also wrote something about how being a Korean was more important than being a soccer player. Both tweets have been interpreted as defenses of his monkey act.
This morning’s Sankei Sports newspaper states that there is a possibility that FIFA could deal out some kind of punishment to Ki if it is ruled that his actions were indeed meant to insult the Japanese.