ORANGE BEACH, Ala. - The Coast Guard has demanded that BP step up its efforts by the end of the weekend to contain the oil gushing into the Gulf of Mexico, telling the British oil company its slow pace in stopping the spill was becoming increasingly alarming as the disaster fouled the coastline in ugly new ways Saturday.

The Coast Guard sent a testy letter to BP's chief operating officer that said the company urgently needs to pick up the pace and present a better plan to contain the spill by the time President Obama arrives on Monday for his fourth visit to the coast.

The letter, released Saturday, follows nearly two months of tense relations between BP and the government and reflects the growing frustration over the company's inability to stop the largest environmental disaster in U.S. history.

The dispute escalated on the same day that ominous new signs of the tragedy hit the beaches of Alabama. Waves of unsightly brown surf crashed onto the shores in Orange Beach, leaving stinking, dark piles of oil that dried in the hot sun and extended up to 12 feet from the water's edge as far as the eye could see.

It was the worst hit yet to Alabama beaches. Tar-like globs have washed up periodically throughout the disaster, but Saturday's pollution was significantly worse.

"This is awful," said Shelley Booker of Shreveport, La., who was staying in a condominium with her teenage daughter and friends near the deserted beach about 100 miles from the site of the spill.

Scientists have estimated that between 40 million gallons and more than 100 million gallons of oil have spewed into the gulf since a drilling rig exploded April 20, killing 11 workers. The latest cap installed on the blown-out well is capturing about 650,000 gallons of oil a day, according to estimates.

On Saturday, Obama reassured British Prime Minister David Cameron that his frustration over the spill was not an attack on Britain. The two leaders spoke by phone for 30 minutes.

Cameron has been under pressure to get Obama to tone down the criticism, fearing it will hurt the millions of British retirees holding BP stock.

Cameron's office said that the prime minister told Obama of his sadness at the disaster, and that Obama said he recognized that BP was a multinational company and that his frustration "had nothing to do with national identity."