A highly anticipated test designed to measure pressure within BP's ruptured Gulf of Mexico oil well began Thursday after a delay caused by leaking equipment.

A short time later, BP Senior Vice President Kent Wells announced that for the first time in almost three months, no oil was flowing into the Gulf. This was part of the test, as BP measures pressure in the well to see how it's holding. Higher pressure readings mean the well is containing the oil, while lower pressure means some is leaking out.

The data is being particularly closely scrutinized at six-hour intervals, so a key time will occur later Thursday night, after the first six hours.

The "well integrity" test could end after six hours, if the results are disappointing. But it could go on for 48 hours. The longer it goes, the better indications are that the well is holding with a custom-made sealing cap.

BP cautioned that the oil cutoff, while welcomed, won't go beyond the 48 hours. Valves will open after that to resume siphoning oil to two ships on the surface, the Q4000 and Helix Producer, as government and BP officials assess the data and decide what to do next.

"It felt very good to see no oil going into the Gulf of Mexico," Wells said in a briefing. He said company officials are "obviously very encouraged" but they are "trying to maintain a strict focus" on remembering the whole purpose of the test, which is to gather data and decide how to proceed.

"I don't want to create a false sense of excitement," he said. "We want to move forward and make the right decisions."

BP Chief Operating Officer Doug Suttles said later on CNN's "Situation Room" that no leaks were apparent, but he also struck a cautious tone. "It's way to early to celebrate," he said.

"Hallelujah. It's a step in the right direction," said Democratic Sen. Bill Nelson of Florida, who's been outspoken about the damaged caused by the spill. But he noted, "My goodness. This have taken us three months. We've been hurt, and hurt badly."

Louisiana Governor Bobby Jindall said in a statement he was "cautiously optimistic" as the test proceeds. But he said, "Work to revitalize our coast won't be done until our waters and our shores are completely clean and our wildlife, our communities and our coastal industries are 100 percent restored."

Florida Governor Charlie Crist sounded a similar note, stating, "While I am pleased that oil no longer is flowing freely into the Gulf of Mexico, there is more work to do to help families, businesses and communities on the Gulf Coast as they recover from this disaster."

BP's stock jumped on word that the oil flow had been cut off, as part of the test. The stock rose $2.74 a share, more than 7 percent, to close at $38.92.

Earlier this month, it traded below $30 a share.

The stacking cap, lowered in place earlier this week, has never been deployed at such depths or under such conditions and, therefore, there were no guarantees on how well it would contain the oil, BP said.

The integrity test got under way Thursday after BP replaced what is known as a choke line after a leak was discovered the day before when the company first attempted the crucial pressure test, Wells said.

In the test, BP closed off one by one the valves on the cap system through which oil could escape. If at any time the pressure is deemed too low meaning that oil is escaping through another source in the breached well the testing will stop, according to retired Adm. Thad Allen, the federal government's disaster response manager.

Allen compared low pressure in the well to a leaky garden hose that dribbles out water with your thumb pressed hard on the nozzle, Allen said.

If the pressure readings are sufficiently high, the valves on the stacking containment cap could remain closed and signal a beginning of the end to the catastrophe that began when the Deepwater Horizon drilling rig exploded April 20, killing 11 workers and triggering the relentless spill.

Allen said the cap was not designed to permanently shut in the well -- it was meant to move to a four-vessel containment system and assure redundancy in the event of a hurricane. But he said there could be a huge side benefit if the oil can be contained -- a "twofer," as he called it.

Allen said the more permanent solution to the spewing oil remains the two relief wells BP is drilling and expects to have them finished in August.

BP pumped drilling mud into those relief wells to mitigate risks during the pressure testing. The two wells intersect with the Macondo.

Oil recovery was stopped Wednesday ahead of the integrity test but resumed while BP was fixing the problem with the leaking choke line. It was stopped again with the testing under way.