RIO DE JANEIRO - Part one of American swimming sensation Katie Ledecky's historic Olympic treble attempt is complete.

She won the 400m free in a world record time of 3min 56.46sec on Sunday, remaining undefeated in nine individual finals at the Olympics and world championships.

Great Britain's Jazz Carlin won silver at the Olympic Aquatics Stadium in 4:01.23. American Leah Smith took bronze (4:01.92).

Said Ledecky: "I'm pumped. Swimming 3:56 feels really good. I've been so close before. It's pure happiness...I felt good throughout. I had a lot left at the end, I just had more pop on the last lap."

Should Ledecky continue to vanquish all before her in the 200m and 800m free events, she will become the first Olympian to match Debbie Meyer's 1968 feat.

The 19-year-old competed in just the 800m free in her Olympic debut at the 2012 London Games. Since that unexpected victory, she has set 12 world records.

Her latest eclipsed her own mark of 3:58.37 - set at the Pan Pacific Championships in Gold Coast, Australia in 2014.

Ledecky had laid down a marker when she set a new Olympic record of 3:58.71 in the heats earlier on Sunday.