NEW ORLEANS — BP PLC is marshaling equipment and conducting tests today ahead of a new effort to choke off the oil's flow from the seafloor well blew out following an April 20 rig explosion.

Crews hoped that by Sunday they can start the "top kill," which involves pumping heavy mud into the crippled equipment on top of the well, then permanently sealing it with cement.

The procedure has been used before to halt gushing oil above ground, but like other methods BP is exploring it has never been used 5,000 feet below the sea.

That's why scientists and engineers have spent much of the last week preparing for the complex operation and taking a series of measurements to make sure that the mission doesn't backfire.

BP already is sucking up oil through a mile-long tube it managed to insert into the leaking pipe over the weekend.

BP spokesman Mark Proegler said today that the tube is now capturing 210,000 gallons a day but some is still leaking. He would not say how much, but that suggests the leak is bigger than the 210,000 gallons a day the Coast Guard and BP have estimated.

Also heavy, sticky oil from a massive monthlong spill is starting to clog Louisiana marshes on the Gulf of Mexico as another edge of the partly submerged crude reached a powerful current that could take it to Florida and beyond.