RIO DE JANEIRO, Brazil — The Pan American Games do not start until Friday, but the U.S. delegation has already found itself in a controversy after a phrase that Brazilians considered prejudiced and demeaning appeared on a message board at the American media center here.

"Welcome to the Congo!" were the offending words written on a white board, photographed by a Brazilian journalist and published by the principal daily newspaper in Rio, O Globo, last week. The publicity has led to an outpour of anti-American sentiment, which has not been assuaged by an American press aide's explanation that the comments were meant as a reference to a heat wave here and nothing more.

"It's really unbelievable," the mayor of Rio de Janeiro, Cesar Maia, said in a telephone interview Monday. "He's scored a goal for the opposition, right at a time when the image of the United States and its government, for the reasons we are all aware of, is highly unpopular, and the United States is trying to improve that image through sports. The U.S. wants to show it is not an imperial country, and along comes this guy to exacerbate that image."

More than 5,500 athletes from 42 countries and dependencies are scheduled to compete in more than 30 sports during the 15th Pan American Games, which conclude July 29 and are viewed as one of the last significant international events before the 2008 Olympics next summer in Beijing.

The U.S. delegation consists of more than 600 athletes and about 300 support personnel, with Puerto Rico sending a separate delegation of more than 200 competitors.

Brazil spent more than $1.5 billion on preparation and new facilities — including construction of new stadiums, arenas and an athletes village — and was sensitive about any suggestion that its performance was substandard. Brazilian government and civic officials are hoping that a successful staging of the games will strengthen their bids to win the 2016 Summer Olympics for Rio and the FIFA World Cup throughout Brazil in 2014.

In addition, Brazilians have historically reacted with vehemence to anything they have seen as attacks on their dignity and self-respect, especially when Americans have been involved. Ronald Reagan was roundly criticized when he visited Brasilia in 1982 and offered a toast to "the people of Bolivia."

When "The Simpsons" television program broadcast a satirical episode in 2002 portraying Rio de Janeiro as full of wild animals, street criminals and sexually voracious residents, a formal protest campaign was mounted here.

"With respect to your phrase, typical of Americans who have serious problems with world geography, a piece of advice: GO BACK HOME!" one irate Rio resident wrote in a letters column published by O Globo. "You're not welcome here, or in the Congo either."

The U.S. Olympic Committee has responded to the incident by issuing a formal apology "to the people of Brazil and Rio de Janeiro for the regrettable actions."

Copies of the statement, translated into Portuguese by the U.S. embassy, have been sent to the Brazilian Olympic Committee, the games' organizing committee, the Pan American Sports Organization and the mayor.

"We are honored to be guests in Brazil and look forward to what we know will be a spectacular Pan American Games," the statement said. "The actions depicted in O Globo are completely inconsistent with the expectations we have of every member of our delegation and will not be tolerated."

In addition, Kevin Neuendorf, the employee who was photographed working at his desk in front of the message board, "has been dismissed from the delegation and sent home," said Darryl Seibel, a spokesman for the USOC But when asked if the O Globo account that named Neuendorf as the author of the offending phrase was accurate, Seibel declined comment, saying, "We have nothing to add beyond the statement."

Maia, the mayor, said American athletes could probably expect to hear additional boos as a result of the gaffe, especially in sports where the two countries are traditional rivals, like basketball. But he said U.S. diplomats and delegation leaders had acted promptly and deftly to defuse the situation by issuing the apology. He said no additional security for the U.S. delegation would be needed or provided.

"I've met with FBI agents on this, and what they are worried about is symbolic acts of terrorism, like someone trying to bring a plane here or some fanatic throwing a bomb," he said. "They are not worried about this, nor should they be."