The environment ministry says up to 30,000 tonnes of oil flooded into the sea after Israeli jets attacked storage tanks at the Jiyyeh power plant south of Beirut on July 13 and 15.
The spill has affected more than 100 kilometres of the Lebanese coast.
Yacoub al-Sarraf, the Lebanese environment minister, said: "We have never seen a spill like this in the history of Lebanon. It is a major catastrophe.
"The equipment we have is for minor spills. We use it once in a blue moon to clean a small spill of 50 tonnes or so. To clean this whole thing up we would need an armada."
The EU commission said the Lebanese authorities had asked for "urgent" assistance to clean up the oil.
Stavros Dimas, the EU environment commissioner, said: "Wars do cause enormous human suffering as we are witnessing now in Lebanon. But another aspect is also the significant environmental destruction caused by it.
"[The spill] could affect the livelihood and health of the Lebanese and people in neighbouring countries as well as the status of the marine environment in the region."
The government has also asked the UN environmental protection agency to assist in the clean-up operation.
Al-Sarraf said the cost of removing the oil could reach $40-50million.
An Israeli warship damaged by a Hezbollah missile on July 15 may also have spilled oil into the sea, according to the environment ministry.
One of the main problems is that an Israeli air and sea blockade of Lebanon, in place since the war began on July 12, is hampering both the clean-up and the delivery of equipment.
Sarraf said: "To really clean it up we need access to the sea, which we don't have.
"We need more equipment and mobilisation but for that we need the hostilities to end."
Local environmentalists say the marine ecosystem could take years to recover.