COPENHAGEN - The International Olympic Committee has told the four cities vying for the 2016 Summer Games to steer clear of criticizing their rivals.

The official reminder for fair play came after Rio de Janeiro's bid leader said Tuesday that the IOC's Ethics Commission had been informed of comments from Chicago about the Brazilian city's prospects.

Rio and Chicago are competing with Madrid and Tokyo to host the 2016 Olympics.

Chicago Mayor Richard M. Daley suggested last week that Rio's part in hosting soccer's World Cup in 2014 would count in Chicago's favour when IOC members vote Friday.

"We are aware of the issue," the IOC's communications director Mark Adams told the Associated Press in a telephone interview.

"We would remind all cities that they must be careful of what they say. They must not criticize other bids."

Rio has been sensitive about its role in soccer's biggest event ever since the IOC evaluation commission's report said the 2014 World Cup represented a "challenge" to the 2016 marketing and communications strategies.

Asked about Daley's remark, Rio bid president Carlos Nuzman told reporters it was made known to the ethics panel.

"It is not for myself to make any comment about the mayor of Chicago," he said.

Chicago 2016 chairman Pat Ryan defended Daley later Tuesday.

"I think the mayor did not intend to offend Rio," Ryan said. "We're just going ahead with our game plan."