PRESSURE tests will determine whether experts have finally stopped the Gulf of Mexico oil leak, 86 days and 680 million litres after an oil rig explosion triggered America's worst environmental disaster.
Engineers were cautiously optimistic that they had at last brought the leak under control, but are anxiously awaiting readouts of the oil pressure inside the newly capped well before claiming victory.
If the cap is successful, they expect to be able to control the flow and may choose to pipe the oil into ships while awaiting completion of a relief well, due by mid-August at the latest.
''It's a great sight,'' said BP chief operating officer Doug Suttles, but he immediately urged caution, saying the flow could resume. ''It's far from the finish line ... It's not the time to celebrate.''
But one comforting fact stood out: for the first time since the explosion on the BP-leased Deepwater Horizon rig killed 11 workers on April 20 and unleashed the spill, no oil was flowing into the Gulf.
If tests show strong oil pressure in the well it will mean that securing the cap has not ruptured the well's casing beneath the seabed. Low pressure readings, however, would signal that oil is escaping into the rock structure and would force authorities to remove the cap to prevent further damage.
In that case, BP would be forced to resume recovering as much oil as possible through the lines to ships.
The oil stopped flowing early yesterday Melbourne time when the last of three valves in the eight-storey, 75-tonne cap was slowly throttled shut. That set running a 48-hour watch period for engineers to assure themselves that the well was stable. Results would be assessed every six hours, they said.
News of the development just before Wall Street's close lifted BP shares. They added $US2.74, or 7.6 per cent, to close at $US38.92, still well below the $US60.48 they fetched before the rig explosion. The company was believed to be close to offloading assets worth about $US11 billion to pay for the clean-up, which could take years.
US President Barack Obama, whose political standing has taken a hit because of accusations of government inaction, called the development ''a positive sign'' before adding: ''We're still in the testing phase.''
The well has been spewing between 35,000 and 60,000 barrels (4.1 million and 7 million litres) of crude a day into the Gulf, according to a US government-led panel of scientists, inundating sensitive coastal habitats and devastating fishing and tourism across four Gulf states.
The cost of the mishap has been put at several billion dollars, with Louisiana and Mississippi bearing the brunt.
Plugging the well for good will depend on relief wells being drilled simultaneously. The first is within 30 metres of intercepting the well, which taps into the rich Macondo oil field containing an estimated 50 million barrels of oil.
For nearly two months, the world's window into the disaster has been through a battery of BP cameras, known as ''spill-cam''. The constant stream of spewing oil became a fixture on cable TV news and web feeds, which made for a more dramatic conclusion to the complex procedure that capped the well when, suddenly, the leak stopped.
Despite early excitement among Gulf communities, the president of Louisiana's Plaquemines Parish, south-west of New Orleans, offered caution. Billy Nungesser, a fierce critic of BP and the federal government, said the mess was far from over.
''We better not let our guard down,'' he said. ''We better not pull back the troops because, as we know, there's a lot of oil out there, on the surface, beneath it. And I truly believe that we're going to see oil coming ashore for the next couple of years.''
■ The US Senate has invited BP to testify at a hearing investigating last year's release of the Lockerbie airliner bomber.
It has been suggested that BP played a role in winning the release of Abdel Baset al-Megrahi.
In May 2007 BP signed a billion-dollar exploration agreement with Libya, the same month Britain and Libya signed a memorandum of understanding paving the way for Megrahi's release.