Two people have died and more than 17 were injured, after an internal balcony collapsed at a nightclub in the South Korean city of Gwangju early on Saturday, the local fire service says.
Several athletes at the World Swimming Championships were slightly injured, Yonhap news agency reported.
They are from the US, New Zealand, the Netherlands, Italy and Brazil.
Two South Koreans were taken to hospital in a serious condition and later died, Yonhap reports.
The deceased were 38 and 27, and were not competitors at the championships.
A balcony and staircase inside the Coyote Ugly nightclub collapsed at 02:29 on Saturday (17:29 GMT Friday), while about 370 people were inside.
At the time, several water polo teams were at the club, which is near the athletes' village.
A male diver and a female water polo player from the United States team were slightly hurt.
"This is an awful tragedy," said Christopher Ramsey, head of USA Water Polo.
"Players from our men's and women's teams were celebrating the women's world championship victory when the collapse occurred at a public club.
"Our hearts go out to the victims of the crash and their families."
"We were just dancing and then the next minute we dropped five or six metres and everyone started rushing out of the club after that," New Zealand's men's water polo captain Matt Small told Radio Sport.
It was "business as usual and then it literally collapsed beneath our feet," he added.
Members of the Australian water polo team were also in the club.
"All Australian players are safe and escaped without injury," a statement from Water Polo Australia said.
The 2019 Fina World Swimming Championships end on Sunday.
"Fina deeply regrets the situation and sends its best wishes to any victims of this accident." said a statement from the world swimming body.