The death toll from the Ebola virus outbreak has risen to 4,447, with the large majority of victims in West Africa, the World Health Organization (WHO) says.
WHO assistant director-general Bruce Aylward also said there could be up to 10,000 new cases a week within two months if efforts were not stepped up,
But the rate of new infections in some areas has slowed down, he added.
Sierra Leone, Liberia and Guinea have been hardest hit by the outbreak.
There have been 8,914 cases overall, including the fatal cases, and the WHO says it expects this number to top 9,000 by the end of the week.
The WHO estimates its figures by taking the numbers of confirmed cases and multiplying them - from Guinea by 1.5, from Sierra Leone by 2 and from Liberia by 2.5 - to account for under-reporting.
In other developments:
- The UK began Ebola screening at London's Heathrow airport
- A medic working for Sierra Leone's army at a peacekeeping training centre in Freetown tested positive for Ebola
- A "site manager" was appointed at the Dallas hospital where a nurse was infected, with the task of supervising how workers put on and take off protective clothing
- The US Centers for Disease Control and Prevention has set up an immediate response team to travel to any US hospital where Ebola is diagnosed, to hit the ground "within hours"
- A UN health worker, originally from Sudan, died in Germany after contracting the disease in Liberia
- Facebook co-founder and chief executive Mark Zuckerberg said he and his wife, Priscilla Chan, would donate $25m (£16m) to fight Ebola
- A Spanish nurse - the first person to contract the disease outside of Africa - remains in critical condition, although doctors say there are signs of improvement
- Sierra Leone security forces clashed with angry crowds in the capital Freetown who were protesting delays in removing the body of a suspected Ebola victim
Ebola patients treated outside West Africa*
*In all cases but two, first in Madrid and later in Dallas, the patient was infected with Ebola while in West Africa.
- Avoid direct contact with sick patients as the virus is spread through contaminated body fluids
- Wear goggles to protect eyes
- Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated
- People who recover from Ebola should abstain from sex or use condoms for three months
Mr Aylward told reporters in Geneva that the WHO, which is the UN's health agency, was concerned to see the virus was still spreading in the capitals of the three worst-affected countries.
He said the death rate in the current Ebola outbreak was 70%, describing it as a "high-mortality disease".
He said 95% of cases were limited to areas in the "historic epicentre" of the outbreak, where the rate of new infections appeared to be slowing.
However, he stressed that it would be too early to read this as success.
The latest WHO projections suggested there could be between 5,000 and 10,000 cases a week in Guinea, Liberia and Sierra Leone by December.
"It could be higher, it could be lower but it's going to be in that ballpark," he said.
"In certain areas we're seeing disease coming down, but that doesn't mean they're going to go to zero."
'Must do more'
Countries neighbouring the three worst-affected states are "at risk" and it is important for them to prepare for the possibility of Ebola cases, the WHO said.
The head of the UN mission for Ebola response, Anthony Banbury, also issued a stern warning - telling the UN Security Council by video-link from West Africa that in the race to contain Ebola, the virus was winning.
"If we do not get ahead of the crisis, if we do not reach our targets and the number of people with Ebola rises dramatically as some have predicted, the plan we have is not scalable to the size of such a new crisis," he said.
"We either stop Ebola now or we face an entirely unprecedented situation for which we do not have a plan."
Also on Tuesday, US President Barack Obama said that "the world as a whole is not doing enough" to contain the Ebola threat.
He was speaking at the end of a meeting with about 20 US and allied military leaders primarily focused on the threat from Islamic State militants.
He said the US would continue to do its part but added: "Everybody's going to have to do more than they are doing right now."
The White House said President Obama would discuss the Ebola crisis in a video conference on Wednesday with British, French, German and Italian leaders.
Ebola deaths: Confirmed, probable and suspected
Note: figures have occasionally been revised down as suspected or probable cases are found to be unrelated to Ebola. They do not include one death in the US recorded on 8 October.