The United Nations Environment Programme (Unep) has expressed its "grave concern" about oil pollution in Lebanese coastal waters.
An oil slick caused by Israeli bombing of the Jiyyeh power station now covers 80km (50 miles) of coast.
Local environmental groups describe the slick as an "environmental disaster".
Almost as much oil may have entered the water as during the 1989 Exxon Valdez tanker incident in Alaska, which led to widespread ecological damage.
The UN and other international organisations are assisting the Lebanese government as it attempts to contain thousands of tonnes of oil.
"What we have here is equivalent to a tanker sinking"
"The Lebanese government has requested international assistance from the UN, and we stand ready to do all we can," said Unep Executive Director Achim Steiner.
A number of Mediterranean countries are contributing equipment and personnel.
But according to the Lebanese environment ministry, "minimal amounts of dispersants, booms, adsorbents, and skimmers are readily available".
The incident began with Israeli raids on the Jiyyeh power utility 30km (19 miles) south of Beirut between 13 and 15 July.
Initial reports indicated that 10,000 tonnes of heavy fuel oil had escaped from damaged tanks, but the eventual total could be 35,000 tonnes.
By comparison, spillage from the Exxon Valdez accident totalled just under 40,000 tonnes of crude oil.
Fishing and tourism
"What we have here is equivalent to a tanker sinking, and 20,000 to 30,000 tonnes reaching the shoreline," said Berj Hatjian from the Lebanese environment ministry.
"We've had it immediately rushing into the sea from the beach line," he told BBC News.
The Malta-based Regional Marine Pollution Emergency Response Centre (Rempec) for the Mediterranean, which is advising the Lebanese government, says "a small quantity of tar balls" also reached the Syrian coast further north.
A coalition of environmental groups declared the Jiyyeh spill "one of the worst environmental crises in Lebanese history".
The group Green Line says that some of the oil has settled on the sea floor, threatening areas where tuna spawn.
It also says that slicks on beaches will prevent baby turtles from reaching the sea after they hatch.
The green turtle, whose eggs hatch in July, is an endangered species.
Unep agrees that the oil is a significant threat to some Mediterranean wildlife, but also says the slick could compromise livelihoods when the current conflict ends.
"Firstly our thoughts are with the suffering of the civilian population," said Mr Steiner.
"But we must be concerned about the short and long term impacts on the marine environment, including the biodiversity upon which so many people depend for their livelihoods and living via tourism and fishing."
Richard.Black-INTERNET@bbc.co.uk
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