"Now, let me not be misleading. What you might see is oil being replaced by mud out of the top of the well," he said, "and as long as there is flow, you can't be sure that it was stopped."
The pumping began at 2 p.m. ET, said the oil company. Green-tinted live video from the gulf floor showed the pumping apparatus drifting underwater. Engineers said they were not sure the pictures would clearly show whether the leak had been stopped.
BP was leasing the Deepwater Horizon drilling rig when it exploded April 20, killing 11 workers and triggering the spill. At least 7 million gallons have escaped into the Gulf. Oil has begun coating birds and washing into Louisiana's delicate wetlands.
On Thursday, Obama will announce new measures the federal government will take to try to prevent any future BP oil spills, administration officials said.
Administration officials also said the government will make changes to the way it allows offshore drilling, including new measures for the permitting process, new safety requirements for offshore rigs, and what was described as "strengthened" inspections for drilling operations.
The changes will be announced after the president receives a report from Secretary of the Department of Interior Ken Salazar on the safety issues that have arisen as part of the investigation into the Deepwater Horizon explosion on April 20. The 30-day review was ordered by the president after the explosion and is technically due Friday, though Salazar will turn it in on Thursday.