The top kill procedure has never before been attempted at these depths and its ultimate success is uncertain, the British oil giant said in a statement on Friday.
President Barack Obama is set to visit the Louisiana coast as BP battles deep on the sea floor to stem a flow of oil that has permeated wetlands, closed a lucrative fishing trade and angered locals recovering from 2005's Hurricane Katrina.
The London-based company said attempts to halt the spill by pushing heavy fluids known as drilling mud into the well may continue for another 24 to 48 hours and repeated that in the event of failure the equipment was already in place to try an alternative remedy.
The total financial cost of the response in the five weeks since a rig explosion killed 11 workers and unleashed the oil from a well head one mile down now stands at $930 million, up from a $760 million estimate on May 24, BP said.
(Reporting by Paul Hoskins; editing by Matt Scuffham)