BP has successfully installed a new sealing cap on the leaking Gulf of Mexico oil well, company officials say.
It is hoped the new cap will stop the leak and capture all the oil before it can pour into the sea.
But BP warned the cap system had never been deployed at such depths and said "its efficiency and ability to contain the oil and gas cannot be assured".
It also said that the two relief wells being drilled remain the only way permanently to seal the leak.
That process is due to be completed by mid-August.
Meanwhile, the US government issued a new deep-water drilling moratorium to replace one struck down in court.
The moratorium was reworked by the US interior department in the hope of gaining the approval of a federal court that rejected the Obama administration's earlier ban, saying it was too broad.
The new moratorium applies to any deep-water floating drilling facility but is not based on the water depth where drilling occurs. It will end by 30 November or sooner.
It is unclear whether a federal court in Louisiana will look at the new moratorium more favourably.
Pressure tests
On Monday evening, Adm Allen hailed "significant progress" on the new well cap and live underwater footage showed the device being placed on top of the well.
The old cap, which it replaces, managed to contain only about half the oil escaping from the wellhead.
Adm Allen said BP would carry out tests to determine the integrity of the leaking well on Tuesday morning.
"The measurements that will be taken during this test will provide valuable information about the condition of the well below the sea level and help determine whether or not it is possible to shut the well for a period of time," Adm Allen said in a statement.
During testing, engineers will close one or more valves on the cap mechanism, shutting down the collection of leaking oil for six to 48 hours.
Meanwhile, pressure readings will be taken to make sure there are no other leaks from the well.
The news of progress on the cap came after a visit to Florida by First Lady Michelle Obama, who urged tourists not to abandon the Gulf Coast.
"It's important for the rest of the country to know that these places are just as vibrant and just as beautiful as they've always been," she said in Panama City, Florida.
"And folks here in Florida and across the Gulf Coast are still depending on visitors and tourist dollars to put food on their tables and to pay their mortgages and to send their kids to college."
'Operating mistakes'
The presidential commission set up to investigate the oil spill also began its hearings on Monday.
At the hearing, Larry Dickerson, president of a drilling company not involved in the Deepwater Horizon incident, said the disaster was "the result of reckless operating mistakes".
President Barack Obama, who has visited the area several times, has labelled the spill the nation's worst-ever environmental disaster.
Coastal communities from Florida to Alabama rely on fishing and tourism, and many people believe the spill will wipe out their livelihoods.
BP says it has spent $3.5bn (£2.3bn) on the response effort so far.
In addition, the firm has also set aside a $20bn fund to pay for the clean-up operation and other costs.
The explosion of the Deepwater Horizon rig in April killed 11 people. Since then, thousands of barrels of oil have been spilling into the the sea every day.
