Rio 2016 organisers have issued an apology to Ryan Lochte and his United States swimming team-mates after they were held up at gunpoint on Sunday.

Lochte won gold in the men's 4x200 metre freestyle relay last week to take his impressive haul of Olympic medals to 12, but his Rio experience turned sour when he was involved in a harrowing incident at the weekend.

The 32-year-old was one of four American swimmers stopped by armed men and revealed he had a gun held to his head and was robbed after attending a party they were invited to by Brazilian swimmer Thiago Pereira.

That was the latest in a series of security incidents during the Games and Rio 2016 communications director Mario Andrada on Monday spoke of his regret that the athletes had been subjected to such an ordeal.

"Everybody saw the confirmation by the US swimmers that indeed they were held up when returning to the Olympic Village. We regret the violence has got so close to the athletes," he told reporters.

"We have requested the security authorities that they need to make sure everybody is safe everywhere in the city.

"We apologise to those involved and regret that violence is an issue in these Games.

"We haven't got an update. The police have reached some of the athletes yesterday. The police are looking for the cab driver who seems to be having more information about this."

International Olympic Committee spokesperson Mark Adams defended his initial denial that the robbery had taken place.

"I contacted USOC [USA Olympic Committee], they told me the story was not correct. I reported it was not correct," Adams said.

"I was asked to get an update from USOC, I got an update from USOC, I gave you USOC's update and there was another one after that."