American swimmer Michael Phelps insists he will not be targeting a sixth Olympics in 2020 after winning a 27th Olympic medal in a glittering career.

The 31-year-old is the most decorated Olympian of all time but looks set to call it a day after the Games in Rio finish.

"I'm not going four more years and I'm standing by that," he said after claiming silver in the 100m butterfly final.

"I've been able to do everything I've ever put my mind to in the sport. I'm happy with how things finished."

"No more. This is it. I said it a bunch before. But I'm not doing it. I swore in London I wasn't coming back and this is final. Were the papers here, I'd sign them tomorrow."

Phelps was beaten into second by Singapore's Joesph Schooling in the final, but has already claimed four gold medals since the Games started last week. 

"I guarantee he will be there,'' team-mate Ryan Lochte told NBC after the race. "I think so. I really think so. So Michael, I'll see you in Tokyo."

Phelps still has one more race to run (or swim rather) in the early hours of Sunday morning in the 4x100m medley relay.