China's Sun Yang, centre, holds up his gold medal with silver medalist Australia's Mack Horton, left, and bronze medalist Italy's Gabriele Detti after the men's 400m freestyle final at the World Swimming Championships in Gwangju. Photo: Lee Jin-man/AP Photo

GWANGJU  Sun Yang proved unbeatable again in the men's 400-metre freestyle at the world swimming championships on Sunday as the controversial Chinese claimed yet another gold.

Sun moved into the lead at the halfway point of the opening swimming final of the championships and was never in danger of being caught. He won in 3 minutes 42.44 seconds with his Australian rival Mack Horton moving through the field in the final 50m to take silver in 3:43.17.

Italy's Gabriele Detti took bronze, nudging Lithuania's early leader Danas Rapsys into fourth.

Sun is competing despite a case at the Court of Arbitration for Sport over doping test samples being smashed in a confrontation with officials last year.

He received a warning from the ruling body FINA but the World Anti-Doping Agency wants the case heard at the CAS which won't happen until after the worlds.

Sunday's victory is the 27-year-old Sun's 10th overall at the worlds and fourth straight over 400m.

The 2016 Olympic champion Horton has previously labelled Sun, who once served a secret three-month ban for doping, a drug cheat.

Later American superstar Katie Ledecky will aim for her first gold at the Nambu University Municipal Aquatics Centre in the women's 400m free while there are also men's and women's 4x100m freestyle relay finals.