President Barack Obama will announce a major military-led surge in U.S. aid to fight the "unparallelled" Ebola epidemic in West Africa later Tuesday, with as many as 3,000 troops to help organize, train new health care workers and build treatment clinics.

The Defense Department will divert $500 million for the effort, which will include building 17 treatment centers with 100 beds apiece in Liberia, dedicating 10,000 sets of personal protective equipment and distributing supplies such as disinfectant and hand sanitizer to help 400,000 families protect themselves and care for sick family members.

The World Health Organization, which has been pleading for help for months, says the epidemic is "unparalled in modern times." The official death toll has climbed to 2,500 with at least 5,000 infected. And now it will take $1 billion to control it, the United Nations said.

"The amount for which we requested was about $100 million a month ago and now it is $1 billion, so our ask has gone up 10 times in a month," Dr. David Nabarro, senior U.N. coordinator for Ebola, told reporters in Geneva. "Because of the way the outbreak is advancing, the level of surge we need to do is unprecedented. It is massive," he said.

"The Ebola epidemic in West Africa and the humanitarian crisis there is a top national security priority for the United States," the White House said in a statement. Obama will announce details when he visits the headquarters of the Centers for Disease Control and Prevention in Atlanta. He first hinted about the move in an exclusive interview on “Meet the Press with Chuck Todd".

Senior administration officials, who asked not to be identified because they don't want to upstage the president, said they believe the surge will start to turn the epidemic around. WHO, local leaders and aid groups in West Africa have all said the epidemic is raging out of control, and top WHO officials visited Washington earlier this month to beg for more help.

So does Medecine Sans Frontiers (Doctors Without Borders)

"Today, in Monrovia, sick people are banging on the doors of MSF Ebola care centers, because they do not want to infect their families and they are desperate for a safe place in which to be isolated," the group's international president, Dr. Joanne Liu,said at a United Nations special briefing on Ebola Tuesday.

"Tragically, our teams must turn them away. We simply do not have enough capacity for them. Highly infectious people are forced to return home, only to infect others and continue the spread of this deadly virus. All for a lack of international response," Liu added.

WHO says as many as 20,000 people could be infected in the coming months. One of the senior administration officials said the number could spike to hundreds of thousands of people.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The ramp-up should create a "backbone" that will give other countries the confidence to send in supplies and money to help, the officials said. A U.S. general will coordinate command and control efforts, but the officials said everything will be synced with international groups such as WHO and with local governments. They declined to say it amounted to a U.S. takeover of the aid effort.

The U.S. Africa Command will train as many as 500 new health care workers a week for six months and longer if needed. WHO and aid groups such as Medecins Sans Frontieres (MSF, or Doctors Without Borders) have said medical staff are by far the greatest need.

"U.S. Africa Command will establish a regional intermediate staging base to facilitate and expedite the transportation of equipment, supplies and personnel," the White House said in a statement.

"Just as the outbreak has worsened, our response will be commensurate with the challenge."

Some groups questioned whether even this surge would be enough. "Despite new U.S. goverment resources for Ebola, I expect there will still a huge shortage across the region. Will other governments follow suit?" Michael Stulman, a spokesman for Catholic Relief Services, asked via Twitter. Stulman also asked whether the U.S. push did enough for Sierra Leone and Guinea.

"As of today, MSF has sent more than 420 tonnes of supplies to the affected countries. We have 2,000 staff on the ground. We manage more than 530 beds in five different Ebola care centres. Yet we are overwhelmed. We are honestly at a loss as to how a single, private NGO (non-government organization) is providing the bulk of isolation units and beds," Liu said.

Eventually, the U.S. will supply hundreds of thousands of the needed sets of personal protective equipment — which include full body suits, gloves, boots, goggles and masks to protect health care workers from the virus. Families will get smaller kits.

"In partnership with the United Nations Children Fund, the Paul Allen Family Foundation, and other key partners, we will immediately target the 400,000 most vulnerable households in Liberia," the White House said.

"As part of this effort, this week, USAID will airlift 50,000 home health care kits from Denmark to Liberia to be hand-delivered to distant communities by trained youth volunteers."

Ebola is spread through bodily fluids, and people must be in direct contact, so doctors, nurses and other caregivers, including family members and people preparing bodies for burial or disposal, are at highest risk. Experts say good infection control can prevent its spread.

In a sad acknowledgement that many more deaths will come before the epidemic is ended, the U.S. will also send 5,000 body bags.

The U.S. has already promised $175 million to help fight the epidemic in Guinea, Sierra Leone and Liberia, and the administration has asked Congress for $88 million more. The government is operating without a proper budget, but Congress can appropriate the money as part of emergency legislation, called a continuing resolution, that keeps the government running.

The plans appear to take on board the many calls for the U.S. to greatly step up its involvement, and also for unorthodox treatment approaches. Samaritan's Purse President Franklin Graham, whose group has played a heavy role in Liberia, told NBC News that he thought hospitals were part of the problem and that standalone isolation units may be a better solution.

And groups such as MSF have said it's "incomprehensible" that the U.S. and other countries haven't done more before now.

The White House defended its efforts, saying it's done more to fight this outbreak than any other disease outbreak.

"Last month, USAID airlifted more than 16 tons of medical supplies and emergency equipment to Liberia, including: 10,000 sets of personal protective equipment, two water treatment units and two portable water tanks capable of storing 10,000 liters each, and 100 rolls of plastic sheeting which can be used in the construction of Ebola treatment units," it said.

The CDC and other U.S. agencies have 100 people in West Africa, and the National Institutes of Health is working to develop drugs and vaccines. "CDC has provided on the ground expertise in the largest international response in its history," the White House said.

Later Tuesday, the Senate Health and Appropriations committees will hold a joint hearing on Ebola. Speakers will include Dr. Kent Brantly, the medical missionary who was infected with Ebola and survived after a dramatic evacuation; Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases; and Robin Robinson, who heads the Biomedical Advanced Research and Development Authority.