State of emergency declared as slick spreads

A massive oil spill is expected to hit the sourthern US coast after the governor of Louisiana declared a state of emergency.

The US Coast Guard says that five times as much oil as previously estimated is leaking from a well beneath the site of a deadly drilling rig explosion as the slick threatens wide-scale coastal damage for four US Gulf Coast states.

The Coast Guard said that London BP - the owner of the well who is financially responsible for the clean-up - found a third leak in a well 5,000 feet under the sea off Louisiana's coast.

"BP has just briefed me of a new location of an additional breach in the riser of the deep underwater well," Coast Guard Rear Admiral Mary Landry, who is heading the federal clean-up effort, told reporters at a briefing.

Eleven workers are missing and presumed dead after the worst oil rig disaster in almost a decade.

Swiss-based Transocean's Deepwater Horizon rig sank on April 22, two days after it exploded and caught fire while it was finishing a well for BP about 40 miles southeast of the mouth of the Mississippi River.

Landry said that President Barack Obama had been briefed on the situation, and "we have urged BP to leverage additional assets," including possible help from the US Defense Department. The oil slick threatens coastal wildlife refuges, pristine beaches and estuaries in Louisiana, Mississippi, Alabama and Florida.