Mr. Ricci Bitti and other Olympic officials have publicly asserted that there is no Plan B, that the Games will not be moved from Rio. Still, Brazilian officials have reacted to criticism with statements that have not exactly reassured Olympic officials.

Eduardo Paes, the mayor of Rio, told reporters that he and other authorities were “pretty sure” they would deliver on their promises to have the venues ready in time.

Brazil’s sports minister, Aldo Rebelo, lashed out at unconfirmed reports that Olympic officials were considering moving the Games.

“You can be guaranteed it is nothing but a bluff,” Mr. Rebelo said. Pointing to large sponsorship agreements for the Rio games, he said, “I doubt the Olympic Committee wants to lose that business.”

Of all the challenges Brazil faces, cleaning up Guanabara Bay may be the toughest.

Officials vowed to tackle the problem after the United Nations Earth Summit here in 1992 drew scrutiny of Rio’s foul waters. The Rio state government secured more than $1 billion in loans from Japan’s government and the Inter-American Development Bank for cleanup projects, but they have not been even remotely successful, according to environmental experts. The State Environmental Institute in Rio de Janeiro estimates that more than 10 percent of the trash here is not collected, much of it flowing into the bay through canals and degraded rivers.

Vast amounts of raw sewage leak into the waters. Officials set a goal of treating as much as 80 percent of it by the 2016 Olympics, but less than 40 percent is currently treated.

State environmental officials have acknowledged they would fall short of that goal, The Associated Press reported over the weekend, citing a letter sent to federal authorities requesting more funding to battle pollution.