OSLO — As many as 200 oil-stained birds have been found after an Icelandic cargo ship ran aground last week and began leaking fuel inside Norway's only marine natural reserve, authorities said Sunday.

"Yesterday I saw around 50 injured birds... And today I would say I saw maybe 100 to 150 more," Egil Soglo of the Norwegian Directorate for Nature Management told AFP Sunday afternoon.

"We have so far collected around a dozen dead or dying birds that we have put down. We have received the green light to put down more, but have not really started with that yet due to the difficult weather conditions," he added.

The Godafoss container ship was carrying a total of 800 tonnes of fuel when it struck a rock on a well-indicated reef late Thursday near the mouth of the Oslo Fjord shortly after leaving port in the southeastern town of Fredrikstad for Helsingborg in southern Sweden.

The ship, which was carrying 439 containers, including two filled with 12 tonnes of dynamite, was on Sunday still leaking oil into the Ytre Hvaler marine park, home to a wide variety of sea birds, marine life and large cold-water coral reefs, authorities said.

"The cold weather is challenging the oil clean-up after the Godafoss accident. Ice, fog and temperatures down to around 20 degrees Celsius below freezing (-4 degrees Fahrenheit) are complicating the work," the Norwegian Coastal Administration (NCA) said in a statement.

It noted that ice was drifting into the oil booms and filling them.

The NCA said the fuel had reached the fragile shoreline in several places, and that it was still flying helicopters and planes over the area to get a better idea of the extent of the damage.

The dynamite had by Sunday morning meanwhile been removed from the ship.

The Ytre Hvaler park, which was created in June 2009 and stretches across 354 square kilometres (137 square miles), is Norway's only marine natural reserve and is located not far from the Swedish Kosterhavet marine national park.

The Swedish coast guard, which was helping with the cleanup, said late Saturday about 50 cubic metres (1,766 cubic feet) of oil had so far been removed.