BP began a pressure test on the Macondo well after a new cap choked off the flow of oil into the Gulf on Thursday for the first time since the April 20 rig explosion of the rig.
The test is intended to show whether the blowout damaged the piping and cement inside the well, which could allow oil and gas to leak out the sides and seep up through the seabed.
"We're feeling more comfortable that we have integrity" of the well, said Kent Wells, BP senior vice president of exploration and production.
But the test could last beyond the original 48-hour deadline set for Saturday afternoon because pressure was rising very slowly, Wells said.
He added that there was always the provision that the test could be extended. "The longer this test goes, the more confidence we have."
Once tests are complete, BP plans to siphon up to 80,000 barrels (12.7 million liters) of oil a day and send it a mile (1.6 km) up to waiting ships.
Retired Coast Guard Admiral Thad Allen, the US government's point man on the spill, has final say on when the test will end and BP's next course of action, Wells said.had reached 6,745 pounds per square inch and was rising about 2 psi per hour, Wells said.
Allen and BP want pressure to hit and sustain 7,500 psi or more, which would indicate all the oil and gas was flowing to the top with no breaches. Increased pressure would indicate the well could hold back all the oil flow if ships siphoning off the crude had to disconnect and move away in advance of a hurricane.
Pressure beneath 6,000 psi would indicate a possible leak.
US President Barack Obama warned on Friday that more work was needed before the well could be considered fixed.
"We won't be done until we actually know that we've killed the well and that we have a permanent solution in place. We're moving in that direction, but I don't want us to get too far ahead of ourselves," Obama said at the White House.
Obama is under fire to push London-based BP to permanently plug the leak and clean up an environmental and economic mess across all five states on the Gulf of Mexico. The spill, the worst in US history, has cut into multibillion-dollar fishing, tourism and drilling industries.
Last month Obama coaxed BP to establish a $20-billion fund to cover damage claims from the spill.
The crisis has complicated US relations with close ally Britain.
British Prime Minister David Cameron is set to visit the White House on Tuesday amid a simmering controversy over BP's possible involvement in negotiating the release of a Libyan man convicted of the 1988 bombing of a US passenger jet over Lockerbie, Scotland.
The US Senate Foreign Relations Committee plans to ask BP officials to testify after the company said it had lobbied the British government in 2007 over a prisoner transfer agreement with Libya.
Cameron said on Friday he would stress how important a "strong and stable" company is to both nations.
Many Britons believe Washington is treating BP too harshly, to the detriment of British pension funds and other investors who have big stakes in the company.
US lawmakers also are considering a range of new rules that could impose tougher safety regulations on offshore drilling or bar companies like BP from new offshore exploration leases. Debate is set to step up this week as Congress is slated to debate an energy bill.
BP still expects to drill a new well by early August to intersect the ruptured one and seal it with cement.
Investors remained cautious on BP's latest effort, however, as several previous attempts failed to plug the leak caused by the explosion that killed 11 men.
Shares in the company, which fell about 50 percent in the first two months of the crisis but have gained 40 percent since late June, were down nearly 5 percent on Friday as part of a broader sell-off on the New York Stock Exchange.
Estimates vary widely on BP's total costs — from $40 billion to $100 billion — which will run on for many years as lawsuits wind their way through courts.
Investors welcomed reports that BP was moving closer to sealing the first deal in its planned $10 billion sale of non-core assets to help pay cleanup costs.
The company and bankers were finalizing details of the asset sales, including some US interests to Apache Corp, CNBC and the Financial Times said.
Still, local residents worried that fierce storms, which often plague the region in the summer, could complicate efforts.
"I think Mother Nature will take care of it eventually," Philip Simmons of Empire, Louisiana, said of the clean-up. "But if you get a hurricane or really bad weather then we're really in trouble."
