Allyson Felix clinched a record sixth career Olympic gold as the United States won the 4x400m relay for a sixth straight Games.

Reigning 400m world champion Felix, 30, took home the baton on the final leg to cross in 3min 19.06sec.

Jamaica took silver in 3:20.34 with Britain claiming bronze in 3:25.88.

The win marks a 20-year unbeaten reign in the event for the United States.

They have not lost a 4x400m relay since finishing in silver behind the unified team of former Soviet Union states at the 1992 Barcelona Games.

On Saturday at the Olympic Stadium they were in command from the outset, with 22-year-old Courtney Okolo handing the Americans the lead after a solid opening lead.

Natasha Hastings extended the advantage after the second leg, but a superb third leg by Jamaica's Shericka Jackson, who closed the gap on Phyllis Francis set up a nervy finale.

Felix set off calmly on her anchor leg pursued by Novlene Williams-Mills, the 34-year-old veteran who underwent a double mastectomy following the 2012 Olympics after being diagnosed with breast cancer.

But just when it looked as if Williams-Mills might pull off a shock, Felix coolly kicked it up a gear and accelerated to victory.

Felix, who 24 hours earlier had helped the USA win 4x100m relay gold, has now won more track gold medals than any other woman athlete in history.