By Kieran Mulvaney

LAS VEGAS (Reuters) - American Danny Garcia knocked out Britain's Amir Khan in the fourth round to add the WBA light-welterweight crown to his WBC super-lightweight title in Las Vegas on Saturday.

Khan, 26-3 (18 KOs), looked the superior fighter early on, showing vastly superior hand speed as he landed right hands behind a long jab and opened up a cut on Garcia's eye with a left hook.

Garcia's defense remained tight and effective, however, and in the third round began throwing thudding shots to Khan's body and near the end of the round he landed a powerful counter left hook that dropped Khan hard on to his back.

The Briton struggled to his feet, but was on shaky legs as the bell rang.

Garcia dropped Khan again at the start of the fourth with a right hand and as he retreated, Garcia gave chase, launching huge punches as he did so.

A left hand put Khan down once more, and although he beat the count, referee Kenny Bayless stopped the fight though the Briton thought the stoppage may have been premature.

"I was a little surprised the ref stopped it," he said. "I thought he was going to let us continue. My mind was clear, and I thought my legs were okay.

"(But) it wasn't my night. I respect Danny. He was countering very well against me. I got a little complacent and he took advantage and he caught me."

Garcia said he had felt the Briton had underestimated him.

"We knew Khan was going to come out fast because he thought I had no power," said Garcia who improved to 24-0 (15 KOs).

"But I waited and stepped up and used the speed and power I have. And it worked."

The loss was Khan's second successive defeat after he lost his WBA and IBF titles on a split decision to Lamont Peterson last December.

Peterson subsequently tested positive for synthetic testosterone, which he admitted he took before the Khan fight, and the WBA reinstated Khan as champion on Thursday.