London (CNN) -- Environmental campaign group Greenpeace has warned that if oil leaks from the stricken cruise liner Costa Concordia it could cause an environmental disaster, threatening marine life including birds, whales and sharks.
At least six people died when the huge ship hit rocks and slid onto its side. The vessel -- containing hundreds of tons of fuel oil -- is now partially submerged off the Italian island of Giglio, which lies inside the Pelagos Sanctuary for Mediterranean Marine Mammals.
The sanctuary was set up 10 years ago in an agreement between France, Italy and Monaco, with about half of it existing in international waters.
Pelagos is a haven for many marine species including the fin whale, sperm whale, bottlenose dolphin, tuna, swordfish and sharks, as well as supporting sea bird populations.
Greenpeace has previously criticized what it called a lack of management of the sanctuary and warned regional governments about ship congestion, particularly in the straits between Corsica and Sardinia.
In a 2010 report the group highlighted the large number of ships in the area and says it has spoken to cruise companies about the risk of pollution from their ships.
"If all this fuel is lost it is going to be a serious disaster," said Greenpeace Pelagos expert Alessandro Gianni.
"Fuel oil is much worse than diesel. It's sticky, very heavy. They must ensure they clean the tanks as fast as possible and then remove the shipwreck," he said.
Italian Environment Minister Corrado Clini told Italian newspaper La Repubblica: "We have to hurry because if the weather changes the situation could get worse. We must protect our natural heritage and landscape."
Clini has already given backing to a plan to restrict cruise ships in the Venetian lagoon and said in an interview with La Stampa that he would consider a levy on passengers to protect the coastline.
Greenpeace is particularly concerned about the Mediterranean fin whales which it says spend the winter off the northern coast of Africa and then migrate to summer feeding ground inside the sanctuary. They are the second longest whale in the ocean after the blue whale, and the International Whaling Commission has previously reported concern about the high number of collisions with ships.
"If there is pollution this could have a big effect on the migration of the whales," said Gianni.
Costa chairman and chief executive Pier Luigi Foschi confirmed that the vessel is carrying 2,300 tons of oil, split between heavy fuel oil and gas oil.
"It is in a sea we want to protect," he said. "We are taking steps to avoid environmental issues."
In a statement, Costa Cruises said: "We have engaged the services of a worldwide leader, a specialized salvage company to develop an action plan and help establish a protection perimeter around the ship."
A company spokesman added: "The main risk is the fuel. We are not aware of any leakage. At the moment we are not aware of any other contents which could be a problem."
The search for survivors and victims was suspended on Monday because the ship began to move.
Greenpeace is concerned that other toxic substance that may be on board -- such as lubricants and paints -- could spill into the sea and get into the food chain.