The offshore oil well at the site of a drilling-platform disaster in the Gulf of Mexico is leaking five times more petroleum than previously thought into the sea, U.S. Coast Guard officials said late Wednesday night.

"It's premature to say this is catastrophic. I will say this is very serious," Rear Adm. Mary Landry said.

A second leak in the well was discovered Wednesday, and scientists from the U.S. National Oceanic Atmospheric Administration now believe that 5,000 barrels a day of oil, up from 1,000 a day, is pouring into the Gulf.

An official at BP, the petroleum conglomerate that operated the rig, said he thinks the new leak hasn't increase the flow rate. Doug Suttle, the company's chief operating officer, held to the 1,000-barrel-a-day figure in a news conference Wednesday night, as Landry stuck to the NOAA estimate.

The Deepwater Horizon oil platform, located about 80 kilometres off the Louisiana coast, exploded April 20, sending giant fireballs into the sky as it went up in flames. A total of 126 workers were aboard at the time; 11 are missing and presumed dead.

The rig collapsed into the Gulf last Thursday, rupturing the platform's wellhead 1,500 metres below the water's surface, sending oil gushing into the water.

As of Wednesday midday, the resulting oil slick measured 160 km by 72 km and was creeping toward the Louisiana shore, threatening an environmental disaster.

Thousands of litres burned

Mere hours before the latest announcement about the second leak, the coast guard had started a test burn of some of the oil.

The coast guard used a 152-metre boom to corral thousands of litres of thick petroleum at the surface, tow it away from the coastline and set it on fire for an hour.

Petty Officer Thomas Blue said burning the oil is better than letting it reach ecologically sensitive areas on the shore.

"They don't go out and light the entire oil sheen on fire," Blue said before the burn began. "In the past these burns have burned about 90 per cent of the oil, and they've found that this is better for the environment," he said.

Wednesday's burn began in the late afternoon, with officials setting fire with hand-held flares to a test area about 50 kilometres east of the Mississippi River delta. The test was deemed a success, but the coast guard didn't start any new burns before nightfall.

Greg Pollock, head of the oil spill division of the Texas General Land Office, said the burning method is effective.

The oil has the consistency of thick roofing tar and when the flames die off, Pollock said, the material that is left resembles a ball of hardened tar that can be removed from the water with nets or skimmers.

"I would say there is little threat to the environment because it won't coat an animal. And because all the volatiles have been consumed, if it gets on a shore it can be simply picked up," he said.

Crews have been trying with submersible robots to activate a shut-off device in the well, but those attempts have been unsuccessful.

In another development, BP said work could begin Thursday to drill a relief well to alleviate pressure in the blown-out well, but the process could take months.

BP said cleanup costs are running at $6 million US a day, and it could require $100 million US to drill a relief well. The total cleanup could cost around $1 billion US.