Humanitarian chief warns capacity in affected countries on brink of collapse as US vows to send in 3,000 troops to fight epidemic

President Barack Obama called the Ebola epidemic in west Africa a potential threat to global security as the White House pledged to send 3,000 troops to fight the worst ever outbreak of the disease in history.

"If the outbreak is not stopped now, we could be looking at hundreds of thousands of people infected with profound political, economic and security implications for all of us," Obama said, speaking at the US Centers for Disease Control and Prevention in Atlanta.

While in Atlanta, Obama met with healthcare professionals from Emory University, where two American aid workers infected with the deadly disease were successfully treated and released last month after being given doses of an experimental drug. Another American infected with Ebola is receiving treatment at Emory while a fourth is receiving treatment at a Nebraska hospital.

Almost $1bn (£620m) is needed to contain the Ebola epidemic raging across Guinea, Liberia and Sierra Leone, which could infect up to 20,000 people if unchecked by the end of the year, the UN warned, as the US pledged to send troops to help contain the world's biggest ever outbreak.

"If not dealt with effectively now, Ebola could become a major humanitarian crisis in countries currently affected," Valerie Amos, the UN humanitarian chief, told reporters in Geneva. The capacity of Guinea, Sierra Leone and Liberia to provide even the most basic necessities was, she warned, "on the brink of collapse".

The stark warning was echoed by other health bodies at an emergency meeting in Geneva on Tuesday, with the World Health Organisation (WHO) emergency chief, Bruce Aylward, saying the outbreak was "unparalleled in modern times".

"We don't know where the numbers are going," he said. He said two weeks ago that when the WHO said it needed the capacity to manage 20,000 cases "that seemed like a lot. That does not seem like a lot today."

The US said it would send 3,000 troops to help tackle the Ebola epidemic.

A regional centre run by the US army in Monrovia, the capital of Liberia, where the outbreak is spiralling out of control fastest, will coordinate efforts to build more than a dozen treatment centres and train thousands of healthcare workers.

More than 2,400 people have died from the virus, for which there is no approved cure in this outbreak. A few cases have been recorded in Nigeria and Sierra Leone. All 26 previously recorded outbreaks have been successfully contained largely by isolating patients but the WHO said cases would continue to rise for at least sixmore months in an epidemic that has jumped borders and erupted in urban areas.

WHO said about $987.8m was needed for everything from paying health workers and buying supplies to tracing people who had been exposed to the virus, which is spread by contact with bodily fluids such as blood, urine or diarrhoea.

Foreign medical teams with up to 600 experts, as well as at least 10,000 local health workers, are needed to stem the outbreak, the global health body has said.

About $23.8m is needed to pay burial teams and buy body bags, since the bodies of Ebola victims remain highly infectious and workers must wear protection suits.

The cost of the US's latest effort will come from $500m allocated for overseas contingency operations, such as the war in Afghanistan, which will instead be reallocated to west Africa and Iraq.

"This humanitarian intervention should serve as a firewall against a global security crisis that has the potential to reach American soil," US senator Chris Coons told the Associated Press.

The Obama administration has also requested an additional $88m from Congress to fight Ebola, including $58m to speed production of the ZMapp experimental antiviral drug and two potential Ebola vaccines. The US classifies Ebola as a bio-terrorist weapon.

Troops will take about two weeks to be deployed, with Barack Obama expected to give further details after a visit to the Centres for Disease Control and Prevention in Atlanta on Tuesday.

Liberia has borne the brunt of the epidemic, recording 1,224 deaths as of 6 September.

Médecins Sans Frontières, the medical charity leading the fight against Ebola, has repeatedly said the international community's response has been "lethally inadequate,". amid a shortage of everything from burial teams to surgical gloves.

"We are honestly at a loss as to how a single, private [non-governmental organisation] is providing the bulk of isolation units and beds," MSF president Joanne Liu said in a speech to the UN in Geneva.

"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," she said.

She called on countries with biological-disaster response capacity, including civilian and military medical capacity, to dispatch specialist medical units to west Africa.

Ruggero Giuliano, MSF medical coordinator in Liberia, said plans were under way to double capacity at the organisation's 200-bed facility before ramping up to 1,000 beds. "There are many days where we need to close the [centre's] gates and leave patients outside. The problem isn't building facilities – it's staffing them. You need experts to run the place, people need to be highly trained."

So far, Cuba and China have said they will send medical staff to Sierra Leone. Cuba will deploy 165 people in October while China is sending a mobile laboratory to speed up testing for the disease. Health workers in Kenama, in Sierra Leone's badly-affected eastern region, said they were using plastic bags amid a shortage of basics such as rubber gloves.

The outbreak has rippled beyond the health sector, prompting spending cuts across the board as all three countries struggle with rising inflation and drops in productivity. Liberia and Sierra Leone's fragile postwar economies will shrink by 3.5% this year, the IMF said, meaning less money is likely to go into vital infrastructure and education projects.