The massive slick prompted the southern US state of Louisiana to declare a state of emergency and the White House to step up its response.
Al Jazeera's Sebastian Walker, reporting from a fishing port south of New Orleans, said fishermen were out in the early hours of Friday, trying to get as much of their catch as they could before the oil slick was expected to hit the shore.
"People have been hearing that they have about one or two days left to fish before the whole operation is shut down," he said.
"There's an atmosphere of extreme concern here, with fishermen trying to get as much as they can before things get even worse."
Barack Obama, the US president, said his administration would use every resource at its disposal, including the military, to stop the spill and help alleviate its impact.
"While BP is ultimately responsible for funding the cost of response and cleanup operations, my administration will continue to use every single available resource at our disposal, including potentially the department of defence, to address the incident," Obama said on Thursday.
BP, the British energy company responsible for the damage caused by the leak, said it was grateful for the offer of help from the military.
Growing oil disaster puts fragile coastal wildlife habitats under threat
"We'll take help from anyone, I mean we welcome the offer from the department of defence, we're working with the experts across the industry," Doug Suttles, chief operating officer of BP's exploration and production unit, said.
"We're going to do everything we can to minimise the impact of this event."
Al Jazeera's Tom Ackerman, reporting from Venice in the state of Louisiana, said that after reaching the coast of there, the oil could potentially also reach the shoreline in the states of Mississippi, Florida and Alabama, requiring a massive clean-up operation.
"BP has the responsibility to do the actual clean-up as well as trying to cap the well which is still spewing oil," he said.
"But the clean-up will obviously need a lot of federal help as well as volunteers, who have been already lining to volunteer their efforts."
The oil leak began on April 20 after an explosion at the Deepwater Horizon drilling rig left 11 workers dead and caused it to sink two days later.
The spill has sparked fears of an environmental disaster along the US Gulf coast, which could threaten wildlife as well as economically important fishing grounds and oyster beds.
In March, the Obama administration agreed to open more of the US coastline to offshore drilling.
Kate Sheppard, the environmental reporter for Mother Jones magazine in Washington, said there was now pressure to change these plans.
"There was already a lot of outrage about this offshore drilling expansion and environmentalists were up in arms, a lot of coastal state legislators, Republicans and Democrats, were pretty upset about this.
"This [oil spill] has added fuel to their complaints. They said this could be catastrophic and now they actually have evidence of just how dangerous it actually can be."
Clean-up crews have been struggling to control the slick, with a fleet of vessels dispatched earlier in the week hampered by strong winds and high seas.
Brandon Blackwell, a US coastguard spokesman in Louisiana, told Al Jazeera that the authorities wanted to "fight this spill as far off shore as possible".
"Of course it will hurt the environment, local economies will be impacted," he said.
Blackwell advised people to stay away from coastline threatened by the oil spill, but also to let the authorities know if they see unattended oil.
At a special White House briefing on the slick, the US coast guard said on Thursday that it was being very aggressive in its response to the spill, but admitted at the same time that it was preparing for the worst case scenario of oil reaching shore.
|The slick threatens a delicate environment of birds and marine life [AFP]
BP engineers working with the coast guard have been testing techniques to set fire to parts of the slick to burn off some of the oil and slow its spread.
Neither the US coast guard nor BP offered any new information on efforts to seal off the underwater well head that has caused the massive oil slick.
Four remote-controlled robotic submarines deployed to the leak site earlier in the week have so far failed to activate a shutoff device, called a "blowout preventer", at the head of the well.
As an emergency back-up, BP engineers are also working to construct a giant dome to place over the leaking well to contain it. Collected oil could then be pumped out of the structure.
Prentice Danner, a spokesman for the US coast guard, said that option will take between two and four weeks, and is so far an untested one.