A German tourist had her right arm severed by a shark as she was snorkelling just 50 yards from White Rock beach in Hawaii.

The 20-year-old woman was transported to the emergency room at Maui Memorial Medical Centre in critical condition, with an additional severe bite to her foot.

"We heard screaming from the water and it was this unbelievable scream like I've never heard before," witness Andree Conley-Kapoi told MauiNow. "I stopped working and I said the only time anybody would scream like that would be if they were being attacked by a shark."

"We saw a number of people saying 'oh my god, look at all that blood,'" witness Josh Craddock said. "I couldn't actually see the blood at that stage. She was maybe 60 or 70 yards out from the beach."

Officials have cordoned off two miles of beaches in Makena, Maui County, following the incident.

It is the seventh shark attack in Hawaiian waters this year and the fourth on Maui.