NEW ORLEANS, Louisiana - A tropical storm barreling towards the Gulf of Mexico oil spill site Friday forced crews to suspend operations and halt work to permanently plug the gushing BP well.

The evacuation of vessels responding to the Gulf of Mexico oil spill could last no longer than 48 hours if forecasts for oncoming Tropical Storm Bonnie are accurate, Admiral Thad Allen, seen here on July 19, said Friday. (AFP/Getty Images/File/Chip Somodevilla) Admiral Thad Allen, the US official overseeing the spill response, said that crews aboard two drilling rigs and a container ship were drawing up thousands of feet of pipes from beneath the sea, while non-essential personnel were being evacuated as Tropical Storm Bonnie took aim at the area.

Officials said a cap that has kept oil from escaping the well since last Thursday would stay in place, after a week of tests suggested pressure would not force oil out through new leaks.

With Tropical Storm Bonnie expected to hit the area on Saturday, Allen said the evacuation would set back efforts to finally "kill" the leaking well by up to 12 days.

But with the safety of workers at the well site a top concern, Allen said the weather had forced crews to collect boom and return ships to shore and some of the 2,000-strong crew responding to spill headed back to land.

"The intention right now is to put the vessels in a safe place so they can return as quickly as possible to resume their operations," he told reporters.

He said officials estimated that "if we abandon the scene, it would be 48 hours before we would be back on."

The oncoming storm has forced a halt to the process of concreting the casing on the first of two relief wells.

Once concrete can be placed and set, a process expected to take up to a week, officials hope to perform a "static kill" to plug the well by injecting heavy drilling mud and cement through the cap at the top.

The final operation to cement the reservoir through a relief well would be expected five to seven days after that.

First Lady Michelle Obama, visiting Pascagoula, Mississippi, promised the US government would not forget those affected.

"This isn't over yet. And this administration is going to stand with the people of the Gulf until folks are made whole again," she said.

Officials ordered crews to begin preparing for Tropical Storm Bonnie on Thursday, after forecasters said the system would affect Florida's Gulf Coast and parts of Louisiana.

Bonnie struck south Florida on Friday. Allen said the storm might be mild enough to allow some vessels on remain at the site of the ruptured well.

"The seismic survey vessels, the acoustic vessels and the vessels operating the ROVs (underwater robots) will stay as long as possible, and if conditions allow it they will remain through the passage of the storm," he said.

But if the ship are forced to depart, engineers will have no real-time information about the state of the wellbore below the sealing cap.

Hydrophones will take recordings, but Allen said the information could only be analyzed after the fact.

"Our only real-time feedback will be aerial surveillance and satellite imagery," he said.

Oil has washed up on the shores of all five US states in the Gulf of Mexico since the BP-leased Deepwater Horizon rig sank on April 22, two days after an explosion that killed 11 workers.

Separately, a former rig worker told federal investigators that an alarm that should have alerted Deepwater Horizon workers to a deadly build-up of gas had been muted months before the April 20 blast.

The system, which uses lights and alarms to warn of fire or high-levels of toxic or explosive gases, had been "inhibited," Mike Williams, the chief electronics technician on the rig, told a hearing looking into the disaster.