Just as U.S. President Barack Obama announced he was sending top administration officials to monitor the massive oil spill in the Gulf Coast, the company at the heart of the disaster said one of the ways it's trying to stem the flow of oil is working less effectively.

John Curry, a spokesperson for BP, told the Associated Press that a 1.6-kilometre-long tube inserted into the leaking well has siphoned about 216,000 litres of oil in the last 24 hours. That's a considerable drop from the 350,000 litres of oil per day the tube siphoned on Friday.

While the company said it expects the amount of oil siphoned to vary each day, the dramatic drop appears to be another setback in the effort to control the worst environmental disaster in the United States since 1989, when the Exxon Valdez spilled nearly 42 million litres of oil into the ocean near Alaska.

More than 22 million litres of oil have spilled into the Gulf of Mexico since April 20, when the Deepwater Horizon exploded and sank off the coast of Louisiana, killing 11 workers.

Despite efforts to control the flow of oil, including placing oil-absorbing booms along the shoreline, the brown ooze has reached Louisiana's coastal wetlands and covered wildlife.

On Saturday, a pelican colony, including numerous eggs, was coated in oil, while other nests were threatened by oil approaching the coastline.

As public anger mounted, U.S. Environmental Protection Agency head Lisa P. Jackson was travelling to the region Sunday, while Interior Secretary Ken Salazar and Homeland Security Secretary Janet Napolitano are scheduled to lead a Senate delegation to the area on Monday to monitor the response to the spill.

White House Press Secretary Robert Gibbs also said Sunday that officials with the Justice Department have travelled to the region to gather information about the spill. In an interview with CBS's Face the Nation, Gibbs would not confirm whether the department has opened a criminal investigation.

The news comes a day after Obama named an independent commission to review the disaster. The president named former Democratic Senator Bob Graham and former Environmental Protection Agency chief William Reilly to co-chair the panel, which is to report within six months.

Meanwhile, BP said it will not be until Tuesday at the earliest that crews will be able to shoot mud into the well to try and stem the leak.

Crews will follow the mud with cement in an attempt to permanently shut off the flow of oil. The measure has never been tried 1,500 metres underwater, so engineers have been practicing on land ahead of time.

"It's taking time to get everything set up," said BP spokesperson Tom Mueller. "They're taking their time. It's never been done before. We've got to make sure everything is right."

With files from The Associated Press