A PE teacher leapt into the ocean to save a young woman's life after her arm was ripped off by a shark.

The 20 year-old German woman was snorkelling off Palauea Beach on Maui Island on Wednesday when she was attacked.

"As soon as we stand on the beach, we hear this blood-curdling scream," said Rick Moore, 57, who teaches at Creekside High School in Irvine, California.

"We look out and there was blood everywhere in the white water around her."

He said he swam out to the woman and "about 10ft from her, I saw her floating on her back, with no arm. It was completely severed from her body."

He pulled her remaining arm around his neck and backstroked 100 yards through strong currents to get her to the beach.

"It dawned on me, I was in danger now," he said.

"The shark is around me and she's bleeding. I start praying out loud, 'God, God protect us.' She said, 'I'm dying. I know I'm going to die.'

"I started crying out to God and I got this burst of strength. I swam toward the shore," said Moore, who is also a pastor.

Moore's friend Nicholas Grisaffi, 61, waded in neck deep to help him pull her from the water.

"I had a pure-white rash guard on but there was very little blood on it, an indication of how much blood she lost," said Grisaffi, a teacher from Laguna Beach, California.

"Pretty much everybody was out of control except me and Rick," he added.

"If we're not there, she's not saved. Nobody did a thing. They just stood there in shock, watching the blood and everything."

The men put the attack victim on a kayak and used it as a makeshift stretcher to carry her up a track leading from the beach.

A police officer then put a tourniquet on her stump and she was taken to Maui Memorial Medical Center.

Joshua Craddock, a 23-year-old from London who was sunbathing on the beach at the time of the attack, hailed Moore's bravery.

He said: "He was pretty heroic and selfless to dive in the water when by this stage she was surrounded by a pool of blood which we could see from the shore."

The hospital has not released the identity of the young woman. The two teachers have visited her, and she is in a stable condition.

"I just can't get the screaming out of my head," Grisaffi said. "The arm didn't bother me. At our age, we've seen a lot."

Authorities closed the beach for 24 hours as they searched for the shark. Its species is unknown.

It was the seventh shark attack in Hawaiian waters this year, and the fourth on Maui, according to official state figures.

There were 11 shark attacks in Hawaii last year, with the last fatal attack in 2004.