Here's how Irma stacks up against the worst U.S. hurricanes

As lawmakers on Capitol Hill Wednesday approved an emergency aid package for flood-ravaged Texas after Hurricane Harvey, Hurricane Irma was bearing down on Puerto Rico and threatening the Florida coast.

The $8 billion in initial relief and rebuilding funds for victims of Harvey, which tore into Texas on Aug. 25, is expected to cover just a fraction of the insurance claims and rebuilding costs. The measure is expected to be approved by the Senate and sent to the White House later this week.

Initial estimates place expected damage at tens of billions of dollars in insured losses. But an estimated four out of five homeowners who suffered catastrophic damage were not covered by federal flood insurance.

Flood claims have already swamped the federal flood insurance program, which is still some $25 billion in debt after a wave of claims from major storms in the last few years. Those claims have escalated in recent decades, thanks in part to ongoing development of vulnerable U.S. coastal regions.

The expected claims from Harvey and Irma will put added financial pressure on the government insurance program.

With maximum sustained winds of 185 mph, Irma is one of the most powerful Atlantic Ocean hurricanes in recorded history.

On Wednesday, the Category 5 storm was wreaking havoc in the Caribbean and barreling toward Puerto Rico and the U.S. Virgin Islands. Officials have warned that the hurricane, which was forecast to weaken somewhat as it made landfall, is expected to leave catastrophic damage in its wake as it bears down on Florida.

WATCH IRMA'S WINDS IN REAL TIME

In Key West, the National Weather Service director said his staff is "very worried about the impact of winds and surge on the Keys."

Director Louis Uccellini said the impact on Florida and the U.S. East Coast depends heavily on when and where Irma makes a "right turn" and heads north. The storm's course will depend on the strength of a low-pressure system over the Great Lakes region.

Though the storm's future track remains uncertain, Irma's price tag could reach $125 billion should a Category 4 hurricane strike Miami directly, according to a note from Credit Suisse. That would correspond with a 1-in-100-year event.

President Donald Trump declared emergencies in Florida, Puerto Rico and the U.S. Virgin Islands.

Florida Gov. Rick Scott on Wednesday activated an additional 900 members of the Florida National Guard to prepare for Hurricane Irma, bringing the total to 1,000. During a stop in the Florida Keys, Scott said he plans to order the remaining 6,000 National Guard members to report to duty on Friday.

Scott said state officials were aware of fuel shortages and were trying to help get gas into the region. The Florida Highway Patrol accompanied gasoline trucks into the Florida Keys on Tuesday night.

Scott warned that Irma is "bigger, faster and stronger" than Hurricane Andrew, which pummeled south Florida 25 years ago and wiped out entire neighborhoods with its ferocious winds.

WATCH: Florida prepares for Hurricane Irma