The Bow Jubail ruptured its hull, pouring 220 tonnes of oil into the harbour on Saturday and officials immediately tried to contain the spill.

Rescue workers have been overwhelmed by the number of contaminated birds.

Volunteers have converged on the area to help clean them. Rotterdam is Europe's biggest port.

On Sunday, hundreds of birds covered in oil were being taken to wildlife shelters in Rotterdam, The Hague and Hook of Holland. However, by Monday the sheer numbers prompted state water company officials to set up a help centre in a car park at Maassluis.

"Swans are pretty big birds. They need a lot of space and a lot of people," said spokeswoman Elske ten Have.

At least 800 birds have been contaminated over a 10km (six-mile) radius, reports say. Hundreds are still thought to be in the water.

"I haven't yet seen a swan untouched by the oil. It's a real catastrophe," adviser Claude Velter told local TV.

Harbour master René de Vries told Dutch TV he could not remember a spill on such a scale in the past decade. "Rotterdam was a clean harbour until Saturday and we want to keep it that way."

Booms were deployed in an effort to contain the spill, but by Sunday it had spread along two key waterways near Rotterdam, the Nieuwe Maas and Nieuwe Waterweg.

The Norwegian company Odfjell which owns the Bow Jubail has said it regrets the spill of heavy fuel oil and has set up an investigation to find out what went wrong.

Although the clean-up operation could take weeks officials said the hot weather in the area was helping to alleviate the problem.