American Olympic champion Hunter Armstrong has set a world record in the men's 50 metres backstroke.

Armstrong posted a time of 23.71 seconds in the non-Olympic event while competing on Thursday night in the International Team Trials at Greensboro, North Carolina.

The meet is selecting the US team to compete at this year's world championships in Budapest.

The 21-year-old, who attends Ohio State, won a gold medal at the Tokyo Olympics last summer when swimming the backstroke leg in the prelims of the 4x100m medley relay.

Still, he was stunned by his time in the 50 back, which eclipsed the mark of 23.80 set by Russia's Kliment Kolesnikov in 2021.

Armstrong established an American record in the morning prelims, then went even faster in the evening.

"It was a complete shock," he said. "My best time before this was a 25.50 and in prelims I was able to throw down a 24.00 and that gave me hope.

"But Kolesnikov is one of the fastest backstrokers and freestylers in the world. To be up there with him is just incredible."