Initial reports indicate 10,000-35,000 tonnes of heavy fuel oil spilled into the Mediterranean and the spill has covered up to 80 km of Lebanon's 225 km coastline, he said.

"Every day that passes will increase the potential damage of this tragic incident," Steiner told Reuters by telephone.

"The physical areas are expanding. The ability to minimise damage on the shoreline will also be reduced."

Lebanon's Environment Ministry has called it the worst environmental disaster to hit the Arab state and says another tank containing some 25,000 tonnes of fuel is still on fire and in danger of leaking or exploding.

Unable to deal with the spill -- the Mediterranean's largest since the tanker Haven dumped 144,000 tonnes of crude near Italy and France in 1991 -- Beirut has asked Syria and Jordan for help in a cleanup it says will cost up to US$50 million.

On its Web site, the ministry showed pictures of beaches and rocks caked in black sludge and said it had reached popular tourist areas north of Beirut.

Steiner, also UN under secretary-general, said he was urgently trying to coordinate with EU states to secure satellite images and land-based reports of the spill to get a clearer picture of its impact.

"Certainly any oil slick like this in the Mediterranean will have serious ramifications for the environment, including marine life," he said.