World Rugby has opened an investigation after South African referee Jaco Peyper appeared to mock sent-off France lock Sebastien Vahaamahina in a picture with Wales fans.
In the picture Peyper's elbow is raised into a fan's chin hours after he sent off Vahaamahina for elbowing Aaron Wainwright in Wales' quarter-final win.
Leading France official Serge Simon said if true the photo was "shocking".
World Rugby is trying to "establish the facts" surrounding the picture.
French Rugby Federation vice-president Simon, a former France prop, said on Twitter: "This photo if it is true is shocking and explanations will be necessary."
World Rugby said: "World Rugby is aware of a picture on social media of referee Jaco Peyper with a group of Wales fans taken after last night's [Sunday] quarter-final between Wales and France in Oita.
"It would be inappropriate to comment further while we are establishing the facts."
Peyper was officiating his 50th Test match as Wales progressed to their semi-final against South Africa in Yokohama on Sunday, 2 November.