(CNN) -- A 120-mile oil slick advanced to within a few miles of the mouth of the Mississippi River on Thursday as authorities scrambled to keep the spill from damaging wetlands along the Gulf of Mexico.
The slick was about three miles off the Louisiana coast on Thursday night, according to Coast Guard spokesman Shawn Eggert.
Oil company BP's ruptured well is at the heart of the spill. State and federal agencies have strung miles of floating booms -- inflatable or foam barriers -- around the leading edge of the shoreline in an effort to contain the spill. Authorities said the spill could begin affecting some areas of the coast overnight.
Efforts to shut down the well have failed so far, and more complicated plans may take weeks. Louisiana Gov. Bobby Jindal on Thursday declared a state of emergency ahead of the oil slick's arrival, warning that it covered as much as 600 square miles of water.
President Obama is sending Department of Homeland Security Secretary Janet Napolitano, Department of the Interior Secretary Ken Salazar and Environmental Protection Agency Administrator Lisa Jackson to Louisiana on Friday to inspect the effort to contain the oil slick, his administration announced Thursday.
Ten wildlife refuges in Mississippi and Louisiana are in the oil's likely path, with the Pass-a-Loutre Wildlife Management Area at the tip of the Mississippi River likely to be the first affected, Jindal announced.
Wildlife conservation groups said Thursday the oil could be a disaster for coastal areas of Louisiana, Mississippi, Alabama and Florida.
Nearly 175,000 feet (about 33 miles) of floating booms have been deployed, with about a half-million more feet being readied, federal officials said.
Officials from a handful of federal agencies have recovered more than 18,000 barrels of an oil-water mix. They have deployed nearly 100,000 gallons of dispersant, which breaks up oil, as of Thursday evening, according to the Department of Homeland Security.
Roughly 1,200 personnel are responding to the oil spill, DHS said.
The latest forecast from NOAA showed the leading edges of the slick reaching the Mississippi and Alabama coasts over the weekend and stretching as far east as Pensacola, Florida, by Monday.
The Louisiana coastline is mostly marsh, and the Louisiana Department of Wildlife & Fisheries is worried that the lowlands will catch and hold oil when the water washes into them, spokesman Bo Boehringer said. The department is advising response teams on where to place the booms to protect wildlife. That includes brown pelicans, Louisiana's state bird, and migratory birds.
"For birds, the timing could not be worse; they are breeding, nesting and especially vulnerable in many of the places where the oil could come ashore," said Melanie Driscoll, director of bird conservation for the Louisiana Coastal Initiative.
"The efforts to stop the oil before it reaches shore are heroic, but may not be enough. We have to hope for the best but prepare for the worst, including a true catastrophe for birds," Driscoll said.
The oil well was ripped open by an April 20 explosion that sunk the drill rig Deepwater Horizon, leading to the presumed deaths of 11 missing men.
Wednesday night, the Coast Guard and NOAA raised their estimate of the amount of oil the damaged well was pouring into the Gulf to 210,000 gallons a day, or about 5,000 barrels.
An effort to burn off part of the oil slick on Wednesday destroyed about 100 barrels, said Doug Suttles, chief operating officer of BP. But the technique "clearly worked," and larger burns are planned when weather conditions make them possible.
"We believe we can now scale that up and burn between 500 and 1,000 barrels at a time," Suttles said.
The well is now leaking from three points, BP said. Under the 1990 oil pollution act, passed in the wake of the Exxon Valdez oil spill in Alaska, the company is required to foot the bill for the cleanup.
In Washington, Obama pledged a robust response and said the military may be called on to assist. Obama told reporters he has been getting regular briefings from top officials in his administration. He said a thorough investigation of the spill is planned.
"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 Defense -- to address the incident," Obama said.
Interior Secretary Salazar, who met Thursday with BP officials, said a federal investigation into the rig explosion is under way.
"Our Joint Investigation with the Department of Homeland Security will have every tool it needs -- including subpoena power -- to get to the bottom of what went wrong," he said in a statement Thursday.
Napolitano declared the spill a crisis of "national significance" on Thursday. That's a move that Coast Guard Rear Adm. Mary Landry said allows the government to pour resources from across the country into the effort.
"If BP does not request these resources, I can and I will," Landry told reporters in New Orleans.
Drilling a relief well -- a second well drilled up to a mile or two away that would enter the leaking well at an angle to help plug it -- will take months, NOAA said.
BP is attempting to deploy collection domes over the leak points to collect oil as it escapes, but getting that system in place could take weeks as well, Suttles said.
BP Group's CEO, Tony Hayward, has cast blame on rig operator Transocean Ltd. for the disaster.
BP Group's CEO, Tony Hayward, has cast blame on rig operator Transocean Ltd. for the disaster. Hayward told CNN that the well's blowout preventer, which he called the "ultimate fail-safe mechanism," has failed to shut down the well as designed. Salazar's office said Thursday that BP is still trying to activate the well's blowout preventer.
Salazar has ordered immediate inspections of all deepwater drilling rigs and platforms in the Gulf of Mexico, including inspections of blowout preventers, he said in Thursday's statement. The inspections, which began Monday, should be complete within seven days, the statement said.
There has been no response from Transocean to BP's comments. The cause of the explosion remains under investigation, and at least one of the victims' families has filed a lawsuit against BP and Transocean, accusing BP specifically of negligence.
CNN's Barbara Starr contributed to this report.