Felix and Merritt win US 400m titles

Allyson Felix and LaShawn Merritt have won the 400m titles at the US Olympic track and field trials.

Allyson Felix and LaShawn Merritt ran the fastest 400m times at the US Olympic trials. (AAP)

Allyson Felix and LaShawn Merritt ran the fastest 400m times in the world this year as they made the US team for the Rio Olympics at the American trials on Sunday.

World champion Felix shook off an ailing right ankle to win the women's 400m in 49.68 seconds to keep alive her bid for a rare 200-400m double in Rio.

Merritt powered home in 43.97 for the men's 400m victory.