NANJING, China  — Fears of the Ebola virus cast a cloud over Saturday's opening of the second Youth Olympics in the eastern Chinese city of Nanjing, after three athletes were barred from competition because of the risk of infection.

The entire delegations from three nations affected by the virus — Liberia, Nigeria and Sierra Leone — will also not be competing.

The games, featuring more than 3,000 athletes aged 15 to 18, started with an evening ceremony attended by Chinese President Xi Jinping, U.N. Secretary-General Ban Ki-moon and other world leaders. The 12-day games feature 28 sports.

Athletes from regions affected by the Ebola outbreak are being prohibited from competing in combat sports due to "health authority guidelines" and aquatic events "based on the inability to completely exclude the risk of potential infection," according to a statement issued Thursday by the International Olympic Committee.

Under that ruling, 16-year-old female judo athlete Mamadama Bangoura and 16-year-old male swimmer Alhoussene Sylla, both from Guinea, will miss the games, along with 16-year-old female Nigerian wrestler Bose Samuel.

The three will be invited back to Nanjing later to take part in another sporting competition, the IOC statement said.

Organizers had been prepared to conduct regular health checks on athletes from the four countries affected — Guinea, Liberia, Nigeria and Sierra Leone — to prevent the potential spread of the virus that has killed more than 1,000 people.

Organizers maintained that the delegations from the four nations were still welcome at the games.

Guinea will be represented by its two remaining athletes, but teams from Sierra Leone and Liberia decided not to travel to China. The Nigerian delegation withdrew after its leaders complained that its athletes were being "isolated" and "quarantined" and not allowed to train alongside other competitors.

The Ebola outbreak in West Africa has also forced qualifiers for football's African Cup of Nations to be moved out of Guinea and Sierra Leone.