Search is on for pipeline leak that may have spilled more than 4m litres of oil into Gulf of Mexico

As much as 4.1 million litres of oil may have been discharged into the Gulf of Mexico from a pipeline system off Louisiana's southeast coast, the US Coast Guard said Tuesday (22 November).

Authorities are still trying to determine the exact location and cause of the leak, officials said during a Coast Guard news conference. None of the oil has reached land so far, though its effect on wildlife is still being investigated.

A US Fish and Wildlife official said two oily pelicans were sighted off the Louisiana coast Saturday, but still appeared active and able to fly.

The oil discharge was discovered amid high winds in the Gulf, which helped some of the oil evaporate and disperse. However, patches were still visible, said Captain Kelly Denning, the Coast Guard's New Orleans sector commander.

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When did the oil leak first start?

The US Coast Guard said the oil was discovered near a pipeline system owned by Main Pass Oil Company, a subsidiary of Houston-based Third Coast Infrastructure LLC. The company did not immediately respond to a Tuesday morning request for comment.

It's unclear when the leak started but it was first reported to the National Oceanic and Atmospheric Administration (NOAA) on Thursday morning. A fly-over of the area spotted an oil slick around five to six kilometres wide.

A 108-kilometre stretch of pipeline was shut down last week as officials worked to pinpoint the location and cause of the leak.

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How much oil has leaked into the Gulf of Mexico?

Local news reported on Friday that pipeline gauges indicated around 1.1 million gallons (4.1 million litres) of oil were lost. Federal officials later confirmed the total could be that high.

The amount is far less than the 2010 BP oil disaster when more than 600 million litres were released in the weeks following an oil rig explosion.

Still, an environmental group described the spill as "huge."

In the last 50 years, there have been at least 44 spills in US waters that have discharged more than 1.59 million litres of oil, according to the NOAA's Office of Response and Restoration. If estimates about this spill are correct, it will be three times this amount.

"From dolphins to birds to rare whales, Gulf animals are under siege yet again from a spill-prone industry that puts profit ahead of everything," the Center for Biological Diversity said in a news release.