USA win women's 4x400m gold by a distance

United States have won gold in the women's 4x400m with a time just shy of the world record, 3:16.87.

The Americans were comfortably in the lead and were about 20 metres ahead when Allyson Felix took the baton on the second leg.

Sanya Richards-Ross took the baton home with a time some four seconds ahead of second-placed Russia.

Jamaica finished just short of the Russians and took bronze with 3:20.95.

Team GB were never really in contention for a medal, although they did finish with a season's best 3:24.76.