The Nairobi-based UN Environment Programme (UNEP) said on Tuesday that the spill poses severe ecological and human threats and is already comparable to the oil tanker accident off the coast of France, with the potential to become far worse.
A statement said: "In the worst-case scenario, and if all the oil contained in the bombed power plant at Jiyyeh leaked into the Mediterranean Sea, the Lebanese oil spill could well rival the Exxon Valdez disaster of 1989."
UNEP chief Achim Steiner said: "We are dealing with a very serious incident and any practical steps are still constrained by the continuation of hostilities.
"While I fully understand the complexity and political implications, many are appalled ... that there has been no on-the-ground assessment to support the Lebanese government, no moves possible towards a clean-up and indeed few practical measures to contain the further spread of the slick."
The Jiyyeh plant was bombed by Israel on July 14 and July 15, a few days into its offensive against Hezbollah in Lebanon.
UNEP said 12,000 tonnes of leaking oil had already polluted more than 140km of the Lebanese coast and spread north into Syrian waters.
The Exxon Valdez spilled 37,000 tonnes of oil into Alaska's Prince William Sound after running aground on a reef on March 24, 1989.
The spill caused massive damage from which some scientists argue the area has yet to completely recover.
UNEP said two environmental experts had arrived in Syria to begin assessing the impact of the Jiyyeh spill, which it said it feared had already affected marine life, particularly tuna and turtles, in the Mediterranean.
Ezio Amato, one of the two UNEP consultants, said: "This oil slick definitely poses a threat to biodiversity."
Earlier on Tuesday in Rome, an Italian environmental agency which monitors the Mediterranean said the spill posed a heightened risk of cancer.
Simonetta Lombardo, of Info-Rac, said the leakage "is a high-risk toxic cocktail made up of substances which cause cancer and damage to the endocrine system.
"It is not oil that has flowed but fuel for power stations. This contains substances such as benzene, categorised as a Class 1 carcinogen."