The International Olympic Committee has demanded an explanation from a Russian tennis official after he made derogatory comments about Serena and Venus Williams.

Shamil Tarpischev, who is president of the Russian Tennis Federation and captain of their Davis and Fed Cup teams, called them "the Williams brothers" on a Russian TV chat show.

He has been fined $25,000 by the Women's Tennis Association (WTA) and banned from any involvement in women's tennis for a year.

The IOC is considering imposing further punishment on Tarpischev.