(CNN) President Donald Trump reiterated his warning that those in the path of Hurricane Irma need to get out of its way in remarks to his Cabinet at Camp David on Saturday.

"My administration is monitoring the situation around the clock, and we're in constant communications with all of the governors, with the state and local officials," Trump said in a video of the meeting released by the White House. "We're doing everything possible to help save lives and support those in need. Again, we've never seen anything like this. Together, we will restore, recover and rebuild. We will do it quickly."

Trump said he approved emergency declarations in Florida, Georgia, South Carolina, Puerto Rico and the US Virgin Islands.

"This is a storm of enormous destructive power, and I ask everyone in the storm path to heed all instructions, get out of its way," the President said, echoing a tweet he posted Saturday evening. "And government officials, I know you're working so hard, you'll never work like this, and I appreciate also your bravery. Property is replaceable but lives are not, and safety has to come first."

Trump also said he plans to ask Congress to expedite tax reform efforts in the wake of damages caused by the hurricane.

"I think now, with what's happened with the hurricane, I'm gonna ask for a speed up," he said. "I wanted a speed up anyway, but now we need it even more so. So we need to simplify the tax code, reduce taxes very substantially on the middle class, and make our business tax more globally competitive. We're the highest anywhere in the world right now."

The President earlier posted a link to state information on the locations of shelters, road closures and evacuation routes.

'Millions' could be without power

Saturday afternoon, Federal Emergency Management Agency Administrator Brock Long warned that millions of Florida residents could be without power after the storm blows through, in some cases for weeks.

"We could see millions of people without power in Florida for multiple days in some areas, maybe weeks, and so I think it's very important to set the expectations of citizens," Long told CNN's Rene Marsh at his agency's headquarters in Washington on Saturday. "This is why we ask and plead with people to be ready for multiple days, and unfortunately this is coming into reality."

Asked for a specific number of those who could lose power, Long said, "You know it's hard to speculate, but some of the estimates could be 5 million people without power based on this, you know, south-to-north trajectory, and it's not just Florida; it's going to be moving into Georgia as well ... over the next five days."

Long made clear that residents living in the hardest-hit areas who decided to stay in their homes despite evacuation warnings will likely not receive immediate aid from federal first responders.

Referring to people in the Florida Keys who don't plan to evacuate, Long said, "You're on your own until we can actually get in there, and it's safe for our teams to support local and state efforts."

He added, "there is no safe area within the Keys, and you put your life in your own hands by not evacuating."

Disaster relief money

Long said it was "hard to say" how long the $15 billion approved by Congress for disaster relief will last.

But he added that the administration and Congress "understand the importance of this mission, and the President's given me all the authority I need to move forward. And as I've been saying, paperwork and money's not going to get in the way of us mobilizing and saving lives."

Long said he has faith that Congress will move swiftly if there's a need for more money.

"I think the Congress will act quickly to make sure we have the emergency funding that we need to go forward," he said. "And the actual damage estimates of not only Harvey, but also Irma takes place over a course of several years, so it's not that all money needs to come down at one time."

Speaking to CNN, Texas Republican Rep. Michael McCaul, who chairs the House Homeland Security Committee, called the amount of money allotted by Congress this week for both Hurricane Harvey and Irma relief efforts a "down payment."

"I would call it a down payment, and that's what it was, and FEMA burning probably half a billion a day," he told CNN. "And when you compound that with Hurricane Irma, with Irma, you can imagine how that cost is going to go up. Congress isn't done with this issue. I fully anticipate another supplemental bill to pay for both Harvey and Irma."