The head of fan group Football Supporters Europe says a lack of segregation at the World Cup is "a risk", and shows Fifa has "lost control of their ticketing".

Most group games so far have been played in front of fans of both countries mingling at venues - in stark contrast to what is normally the case in football.

Ronan Evain, executive director of Football Supporters Europe, told BBC Sport: "The absence of segregation is not normal for a tournament like this.

"What is worrying is that Fifa doesn't really know who has tickets here and there... by pushing so much for people to buy tickets and re-sell them.

"So the possibility - or the risk - to have fans from 'Team A' in the middle of the crowd of 'Team B' is stronger than ever before."

Evain was speaking in Dallas, where BBC Sport saw pockets of Dutch and Japanese fans mixed together when the two teams played each other on Sunday.

"I don't know if there are mitigation measures in place, but the risk does exist," he said.

"Hopefully this kind of situation can be solved by just switching tickets and people moving from one section to another. But there is very little that Fifa can do at this stage because they don't know who owns their tickets.

"It's an additional risk, and I don't think it's been taken into consideration.

"There's so many tickets on the resale platforms - Fifa has zero control with what's happening with these tickets. It's hard to tell what will be the behaviour of the people that control these tickets."

Fifa sources referred BBC Sport to the fact that ring-fenced allocations include tickets reserved for supporters of the Participating Member Associations (PMAs), who are allocated 8% of the tickets for each match in which they take part, and that this is in line with previous tournaments.

Evain also says he is concerned that some fans were prevented from bringing flags into the Dallas Stadium at Sunday's match.

The venue will host England's opening game against Croatia on Wednesday.

"You were not really allowed to bring a flag in, or at least to show it, which is inconsistent with most Fifa rules and regulations, but also what was allowed at previous tournaments. That seems to be closer to what is in place for NFL games," he said.

"At a lot of the stadiums it hasn't been a problem, so it's hard to understand what is the actual policy and what is improvisation by the staff locally with the rules that they now have.

"Most of the flags were removed by the staff. The broader problem - and I think it's a demonstration of how much Fifa has little control over this tournament - is that there's no consistent rule, and when you look at what Fifa has published, there's a code of conduct that is very broad.

"But it never clarified a lot of things, like what sort of symbols are allowed and not allowed? Are you able to bring a flag of your region or city or club? A lot of this is still up in the air, and I think there's a bit of learning by the venues, but also, again, inconsistency."

In its tournament guidance for fans, external, Fifa says that "small flags, banners and posters made of a fire-resistant material are allowed in the stadium. Larger flags, banners, posters or instruments must be approved in advance."

It also prohibits flags "that are of a political, offensive and/or discriminatory nature".

On Monday, Fifa won a court hearing in Los Angeles that means Iran supporters are banned from taking pre-revolutionary flags into their matches, classifying it as a political symbol. However, Evain says some non-political flags have also been confiscated from fans by stadium staff.

"It is just this lack of clarity that is hard. You should know the rules before leaving home, and that's not the case", he said.

"The main issue at this stage is the absence of rules. It's very unclear what is allowed in the stadium. What you can bring, what you can't bring. Your flag, your bag, your drums, your banner", he said.

"What we are probably looking at is a tournament where the rules are going to be extremely different from one venue to another - which is not how it should be.

"Fifa seems to have failed to apply rules across the board. And what we've seen in Dallas, it seems that the staff's understanding of the rules of the tournament has more to do with what is usually allowed in an NFL game than the actual code of conduct implemented by Fifa."

Fifa has been approached for comment.