(CNN) -- Oil appears to have stopped leaking from a ship carrying nearly a half million gallons of heavy fuel that ran aground after striking a reef off New Zealand, officials said Saturday.
The MV Rena, a Liberian-flagged vessel, began leaking oil early Wednesday after running striking the Astrolabe Reef, about 12 nautical miles off the coast of Tauranga, on the North Island.
The slick from the Rena appears to be a thinly spread oil, Maritime New Zealand said in a statement.
National On-Scene Commander Rob Service said dispersants would be made available in the event any thick patches of oil are identified; the dispersant is effective only on thicker concentrations.
The oil slick is moving away from nearby islands, spreading west about two to five miles from the ship, Service said.
"There are darker patches in isolated pockets but they seem to be assisted by yesterday's wind," he said.
Almost all the 1,700 cubic meters (450,000 gallons) of fuel carried by the ship remained within the vessel's hull, Service said Friday in a prepared statement.
He said the operation to remove the oil, led by Svitzer Salvage, would be complex.
"There is no way of doing this quickly," he said.
Some oil-soaked, dead birds have been found near the ship, Maritime New Zealand said.
Four blue penguins and two shag birds were being treated after being rescued Friday afternoon from a nearby island, the government agency said.
About 500 defense force personnel were on standby for a shore cleanup if needed, Maritime New Zealand said.
Officials fear the stricken ship may spill additional oil as its condition deteriorates.
"I think we'd expect that at some point to have further problems and potentially even break up," Transport Minister Steven Joyce told broadcaster TVNZ.
Greenpeace expressed "extreme concern" about the spill and urged the government to avoid using further toxic dispersants.
"This is an unfortunate illustration of just how difficult it is to deal with oil spills at sea," the organization's Steve Abel said. "Even a slow, and relatively accessible, oil spill like this one has clearly stretched New Zealand's response capability to its limits.
"It is also potential disaster for the blue whales and dolphins presently calving in the area, as well as numerous other marine species."
CNN's Karen Smith and Laura Smith-Spark contributed to this report.