The Helix Q4000 vessel, centre, began pumping mud into the damaged Gulf of Mexico oil well Tuesday in a procedure called a static kill. (Gerald Herbert/Associated Press)

BP has begun the long-awaited "static kill" procedure at its damaged oil well in the Gulf of Mexico — a manoeuvre that's aimed at plugging the well for good.

The static kill process followed an earlier "injectivity" test, which saw the well's defective blowout preventer being injected with an oil-like liquid to determine if it could handle the static kill process.

Positive results from that test led BP to go ahead with the static kill at 3 p.m. CT.

Workers collect an oily absorbent boom in Bay Jimmy on the coast of Louisiana on Saturday. (Associated Press/Patrick Semansky)"The aim of these procedures is to assist with the strategy to kill and isolate the well, and will complement the upcoming relief well operation," a BP statement said.

The static kill manoeuvre involves pumping heavy mud, and eventually cement, down the well in an effort to push the oil back down. The whole process could take up to 61 hours, officials said.

The pressure in the well dropped quickly in the first 90 minutes of the procedure, a sign that everything was going as planned, wellsite leader Bobby Bolton said.

"Pressure is down and appears to be stabilizing," he said.

Retired U.S. Coast Guard admiral Thad Allen said Tuesday that even if the static kill is successful, the relief wells that BP has been drilling are still vital to plugging the well.

"This thing won't truly be sealed until those relief wells are done," Allen said.

BP said drilling for one of the two relief wells will resume Thursday. It could be finished later this month.

4.9 million barrels flowed out of BP well

Meanwhile, U.S. officials said Monday that the latest estimates suggest roughly 4.9 million barrels, or 780 million litres, of oil poured out of the well before a temporary cap stemmed the flow on July 15.

"Not all of this oil and gas flowed into the ocean; containment activities conducted by BP under the administration’s direction captured approximately 800,000 barrels of oil prior to the capping of the well," U.S. officials said in a statement.

Local and state politicians, including Louisiana Gov. Bobby Jindal, are warning BP that its responsibilities won't end when the well is plugged.

"Their work is only done when all of the oil is gone from the coast and our water, their work is only done when our wetlands and our coastline are restored to their pre-spill status, so we can go back to our way of life," Jindal said.

Oily marsh grass in Barataria Bay on the coast of Louisiana on Saturday. (Patrick Semansky/Associated Press)

BP and federal officials have managed to contain large parts of the spill through skimmers, oil-absorbent booms and chemical dispersants meant to break up the oil.

The U.S. Environmental Protection Agency, which has been monitoring the environmental impact of the spill, conducted tests on the toxicity of chemical dispersants.

The agency released results Monday indicating that the "dispersant-oil mixtures are generally no more toxic to the aquatic test species than oil alone."

The EPA tested eight dispersants, and the tests confirmed that Corexit 9500A, the dispersant used in the Gulf of Mexico, is "generally no more or less toxic than the other available alternatives."

More than 30,000 people are currently involved in efforts to protect the shoreline and wildlife, U.S. officials said.

The spill began after an offshore drilling rig exploded off the coast of Louisiana on April 20, killing 11 workers. It is by far the largest offshore oil spill in U.S. history.

With files from The Associated Press