Sales of bottled water in Norway have rocketed after Oslo residents were told to boil water because of a Giardia parasite scare.

Seventeen people are being treated in the capital this week.

Hanne Buegel of drinks company Ringnes said the company had increased production by 40% - 200,000 litres - daily since the first reports of infected water emerged on Wednesday.

"We are prioritising hospitals and care homes where the need is greatest. We have put in place much bigger production facilities than normal for Oslo," she said.

Coca-Cola Norway spokeswoman Solfrid Flateby said orders were up by 50%. "That shows what kind of pressure we are under to meet demand at the moment."

Authorities have no idea when the scare will end. They are still investigating how the parasite, which can be fatal for the very young or very old, found its way into the water supply.