BP early Wednesday used robot submarines and a complex maneuver in an attempt to stop the massive flow of crude oil spilling into the Gulf of Mexico.

The company tried to cut into the undersea well's riser pipe, the initial steps toward placing a cap over the well that has spewed hundreds of thousands of gallons of oil into the Gulf of Mexico every day since late April.

However, BP's effort stalled after the blade of a diamond wire saw got stuck, said U.S. Coast Guard Adm. Thad Allen, the federal government's point man on the oil spill.

BP's goal is to free the saw and put in a new blade if needed to finish the second cut later Wednesday, Allen said at a news conference.

The risky "cut and cap" operation involved remote-controlled robots to sever the damaged riser pipe. Engineers then plan to place a custom fit cap over the well stub.

Allen said how well that cap fits and stops oil from gushing depends on how smoothly the riser pipe is cut.

"It's a question of how much precision we can bring to it," he said.

Meanwhile, rust-colored oil washed ashore Tuesday on barrier islands off Alabama and Mississippi as a beleaguered BP tried to stop the continued flow of the largest spill in U.S. history.

Doug Suttles, the company's chief operating officer, told CNN's "John King USA" on Tuesday night that the latest procedure should collect the "vast majority" of the oil if it succeeds.

"We'll be putting the cap assembly, loading that out and sending it to the seabed later tonight," Suttles said. "We should be able to install this tomorrow. And hopefully by late tomorrow or Thursday, we should have this thing operating."

But the operation carries the risk that the flow of crude from the ruptured well could increase by up to 20 percent once the damaged riser is cut away.

The job already has been complicated by pipework around the well that has had to be removed before massive metal shears could be brought to bear, Suttles said.

And the gusher may not be shut down until August when BP expects to complete relief wells that will take the pressure off the one now spewing into the Gulf.

The 5,000-foot-deep well erupted after an explosion and fire aboard the Deepwater Horizon drilling rig on April 20. Eleven people were killed. The rig sank two days later, leaving up to 19,000 barrels (798,000 gallons) of oil pouring into the Gulf, according to federal estimates.

After fouling sections of Louisiana's marshes over the past two weeks, the oil was spreading toward the northeast on Tuesday. Tar balls and patches of reddish-brown "weathered" oil came ashore on Dauphin Island, Alabama, south of Mobile, Alabama, and on Mississippi's Petit Bois Island, off Pascagoula, Mississippi, authorities said.

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