President Barack Obama is expected to outline the action later on Tuesday (local time), his second announcement of a major military operation in two weeks after his declaration that Washington would bomb Islamic State fighters in the Middle East.
The U.S. action, which goes far further than previous offers of aid, won praise from aid workers and officials in the region, but health experts said it was still not enough to contain the fast-spreading epidemic.
The death toll from the deadly fever, which spreads rapidly, causes uncontrolled bleeding and fever and typically kills more than half of its victims, has doubled in the past month to 2,461, mostly in three countries in West Africa. The World Health Organization said a "much faster" response was needed to limit the number of cases to the tens of thousands.
"This health crisis we're facing is unparalleled in modern times," WHO Assistant Director General Bruce Aylward told a news conference in Geneva. "We don't know where the numbers are going on this."
Residents in Liberia, where the U.S. force will be based and Ebola is spiralling out of control, celebrated the news that U.S. troops were coming, recalling a previous military operation in 2003 that helped stabilise the country during a civil war.
However, U.N. officials issued stern warnings of the scale of the task ahead, saying the cost of the response had multiplied tenfold in a month to $1 billion. A previous forecast of 20,000 Ebola cases no longer seemed high, one said, as weak West African healthcare systems have buckled from the strain.
The outbreak was first confirmed in the remote forests of southeastern Guinea in March and has since spread across neighbouring Sierra Leone and Liberia. A handful of deaths have also been recorded in Nigeria.
The disease has so far killed about half those known to be infected. Aid workers say recorded cases likely represent just part of the total, and experts say the mortality rate is likely to rise as more cases are tallied and more victims succumb.
The U.S. plan marks a dramatic increase from Washington's initial response last week, which focussed on providing funding and supplies but drew criticism from aid workers for not deploying significant U.S. manpower.
U.S. troops will now set up a command hub in Liberia, a nation founded by freed American slaves, as well as building 17 treatment centres and training thousands of local health workers. It was not clear when the troops would start deploying.
"This is welcome news. This is what we expected from the U.S. long time ago," Anthony Mulbah, a student at the University of Monrovia, said in the dilapidated ocean-front capital. "The U.S. remains a strong partner to Liberia."
FRIENDS IN TIME OF CRISIS
The disease has swamped weak health systems, infecting hundreds of local staff in a region chronically short of doctors. The WHO has said that 500-600 more foreign experts and at least 10,000 more local health workers are needed.
"It is not enough to provide protective clothing when you don't have the people who will wear them," Ghana's President John Dramani Mahama said during a visit to Sierra Leone.
The U.S. deployment revives memories of Liberia's war years, when Monrovia residents piled bodies of the dead at the U.S. embassy to persuade Washington to send troops. In 2003, a U.S. mission helped African forces stabilise Liberia after 14 years of war, in which some 250,000 people are thought to have died.
Liberia's President Ellen Johnson Sirleaf - who wrote to Obama personally last week to plead for direct U.S. intervention - was due to address the nation on Wednesday. Guinea and Sierra Leone have not yet commented on the planned U.S. deployment.
"We know our friends in times of crisis. The U.S. has always been there for us," a senior Liberian government official told Reuters, asking not to be identified.
The U.S. intervention comes as the pace of cash and emergency supplies dispatched to the region accelerates.
Washington has sent about 100 health officials and committed some $175 million in aid so far. Other nations - including Cuba, China, France and Britain - have pledged medics, health centres and other forms of support.
But critics, including regional leaders and former U.N. Secretary General Kofi Annan, have said the international efforts have so far fallen woefully short of what is needed to slow the spread of the epidemic.
Many neighbouring African countries have shut borders and cancelled flights to the affected countries, making the humanitarian response more difficult. An African Union meeting last week called on countries to lift travel bans but many remain in place.
WINDOW OF OPPORTUNITY CLOSING
Medical charity Medecins Sans Frontieres (MSF), which has some 2,000 staff fighting the disease in the region, said other countries need to follow the U.S. lead. The response had fallen "dangerously behind".
"We need you on the ground. The window of opportunity to contain this outbreak is closing," Dr Joanne Liu, MSF's international president, said in a speech at the United Nations. "We need more countries to stand up, we need greater deployment, and we need it now. This robust response must be coordinated, organised and executed under clear chain of command."
There has been no suggestion that U.S. troops would be involved in efforts to quarantine communities hit by the disease, which have led in the past to unrest. MSF has said foreign troops should not be used for quarantining.
Despite new treatment centres gradually being built in the region, they cannot keep up with the pace of new infections, especially in Liberia, where patients are being turned away from hospitals and infecting others.
The WHO's Aylward said it would take "weeks, if not months" to set up enough facilities to isolate new cases of Ebola, so the response in Liberia would focus on community-level care.
