The UN special envoy on Ebola says the number of cases is probably doubling every three to four weeks and the response needs to be 20 times greater than it was at the beginning of October.
David Nabarro warned the UN General Assembly that without the mass mobilisation of the world to support the affected countries in west Africa, "it will be impossible to get this disease quickly under control, and the world will have to live with the Ebola virus forever".
Mr Nabarro said the UN knows what needs to be done to catch up to and overtake Ebola's rapid advance, "and together we're going to do it".
"And our commitment to all of you is to achieve it within a matter of months - a few months," he said.
He was speaking as the first of a US military force started building a hospital for stricken health workers in Liberia and lawmakers debated whether to grant president Ellen Johnson Sirleaf more power amid the Ebola epidemic.
The arrival of 100 US Marines brings to just over 300 the number of American troops in Liberia.
The Marines and their aircraft will help with air transportation and ferrying of supplies, overcoming road congestion in Monrovia and bad roads outside the capital, said Captain R Carter Langston, spokesman for the US mission.
A priority will be transporting building materials to treatment unit sites. The US has said it will oversee construction of 17 treatment units with 100 beds each.
The US military is also setting up a 25-bed hospital to treat health workers who contract Ebola. Rear Admiral Scott Giberson, the acting United States deputy surgeon general, said the facility would be ready in weeks and would be run by the US military.
"We're in training right now. As you may know, not everybody is fully experienced in seeing Ebola-related care of patients," Rear Adm Giberson said. "We have experience deploying in lots of medical settings. However, this is unique."
The 101st Airborne Division is expected to deploy 700 troops by late October. The US may send up to 4,000 soldiers to help with the Ebola crisis, though officials have stressed that number could change depending on needs.
In a call with reporters on Wednesday, USAID assistant administrator Nancy Lindborg said six treatment units were operational in Liberia. She said about 250 beds had come online in the last 10 days or so, and that more would come online in waves until the end of November.
Liberian state media said the House of Representatives would convene a special session to discuss proposed measures outlined in an October 1 letter that would give Ms Sirleaf the power to restrict movement and public gatherings and appropriate property "without payment of any kind or any further judicial process".
The letter also says she can "limit the right to assembly for any reason".
Ms Sirleaf's government imposed a three-month state of emergency beginning on August 6, and a statement warned at the time that this would involve suspending some rights and privileges.
"I see a kind of police state creeping in," said lawmaker Bhofal Chambers, a one-time supporter of Ms Sirleaf who has since joined the opposition camp.
In August, a quarantine of Monrovia's largest shanty town sparked unrest and was criticised as counter-productive before being lifted. The Committee to Protect Journalists has accused Ms Sirleaf's government of trying to silence media outlets criticising its conduct.
Liberia has been hit hardest by the Ebola outbreak, recording more than 2,200 deaths, according to the World Health Organisation. The total death toll by Wednesday was 3,865, with most other deaths in Sierra Leone and Guinea.
In Mali, a health ministry spokesman said two more people had begun participating in the first phase of a study for a possible Ebola vaccine. Mali has not had any cases of Ebola, but it borders the outbreak zone. University of Maryland researchers announced that the first study of a possible vaccine was under way, and that three health care workers in Mali had received the experimental shots developed by the US government.
"Today, we are at five people vaccinated," health ministry spokesman Markatie Daou said. "We envision vaccinating between 20 and 40 people for this first phase and the results are expected next month."