Sergey Kovalev accused American judges of favouring Andre Ward after the Russian suffered a controversial defeat by decision in their world light heavyweight title battle.

Many neutrals at ringside believed Kovalev -- who scored the fight's only knockdown when he sent Ward to the canvas in the second round -- had done enough to retain his WBA, WBO and IBF crowns.

But the three US judges scoring an attritional bout at Las Vegas's T-Mobile Arena all gave more weight to Ward's disciplined display over the closing stages, when he landed scoring blows as the previously unbeaten Kovalev tired.

All three judges scored it 114-113 in Ward's favour, improving the 2004 Olympic champion's record to 31-0 with 15 knockouts.

Kovalev however was distraught at the loss, which he suggested was influenced by his nationality.

"It's the wrong decision. But I don't want to give my opinion. Everybody is here, witnesses are here, everybody saw what happened," he said. "I'm a guest here in the USA and he's a local, and all the judges are from the USA. I agree they support their boxer but honestly, this is sport. Don't make it like politics."

The tight nature of the contest makes a rematch almost inevitable.

Asked if he would face Ward again, Kovalev replied: "Sure -- and I'll kick his ass."

Earlier, Kovalev had quickly made his superior punching power tell, rocking the challenger with two stiff jabs that appeared to startle Ward towards the end of the first round.

Ward was soon in trouble in the second round, with Kovalev piercing his gloves at will with stinging jabs. With 40 seconds left in the round, Kovalev dumped ward on the floor, softening him up with a left jab before a short chopping right landed flush on his chin.

Ward scrambled to recover his senses and was clinging on for the bell.

Kovalev had the better of the exchanges in the third round, but Ward did well to steady the ship without troubling the Russian.

Ward managed to frustrate Kovalev in the fourth, fighting in clinches and attempting to keep his rival off balance.

But the statistics told the story after four rounds, with Kovalev landing 22 power shots to Ward's seven.

Ward, however, refused to buckle and enjoyed his best period of the fight through the fifth and sixth, sticking and moving to score consistently.

Another good round in the seventh, with Ward connecting with three stiff jabs, drew loud cheers, but Kovalev was still landing shots.

As the fight moved into the later rounds, Ward appeared to be increasingly comfortable, keeping his distance and uncorking several energy-sapping body-shots to the Russian.