Geneva (AFP) - The worst-ever Ebola outbreak has killed more than 2,400 people, the UN said Friday, as Cuba pledged the largest foreign medical team deployed so far in the west African health crisis.

World Health Organization chief Margaret Chan warned the spiralling epidemic of the murderous tropical virus demanded a stronger, faster response from the international community.

In the three hardest-hit countries, Guinea, Liberia and Sierra Leone, "the number is moving faster than the capacity to manage them," she told reporters in Geneva.

As of Friday, 4,784 people had been infected with Ebola and more than 2,400 of them had died, Chan said.

She did not specify if the figures also included Nigeria, which has reported 21 cases, eight fatal, since the deadliest Ebola outbreak on record began in Guinea at the start of the year.

Senegal has also seen one confirmed case of Ebola -- a Guinean student who has recovered.

While hailing that no other cases have yet emerged, WHO cautioned Friday that the country remains a "high risk" of further transmission.

Transmitted through bodily fluids, the tropical virus can cause fever, vomiting, diarrhoea, organ failure and internal and external bleeding, killing more than half of those infected.

There are no licenced treatments or vaccines for Ebola, but the WHO has endorsed rushing out potential cures and hopes a tentative vaccine will be available by November.

- Blood transfusions might help -

In the meantime, the UN health agency has said blood transfusions from people who have survived Ebola can be used to help boost antibody defence in those infected.

Hospital officials said Friday that an American doctor being treated for Ebola in Nebraska had been given such a blood transfusion from another American doctor who recovered from the virus.

Rick Sacra, a 51-year-old Christian missionary doctor infected with Ebola in Liberia, had been upgraded from serious to good condition, after a week of receiving plasma from recovered Ebola patient Kent Brantly and an unnamed experimental drug, they said.

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While experimental treatments raise hope of eventually finding a cure, Chan stressed Friday that "the thing we need most of all is people" on the ground.

She welcomed a pledge from Cuba to send 62 doctors and 103 nurses to Sierra Leone, where more than 500 people have died.

The UN's announcement came as health officials in Sweden said they were conducting tests on a patient with a possible case of the virus.

Results are expected Saturday from the unnamed patient who "comes from a country in west Africa" affected by Ebola, a spokesman told AFP.

The WHO estimates that another 500 foreign health professionals and around 1,000 local doctors and nurses are needed to stop its deadly surge through west Africa.

The Cuban health workers will remain in the region for six months, Cuban Health Minister Roberto Morales Ojeda told the Geneva news conference.

All have "previously participated in post-catastrophe situations," and all volunteered for the mission, he said.

Cuba, which has a long tradition of sending its world-renowned medics to help with situations around the globe, has pledged the biggest team to date to take part in the Ebola fight.

The United States has already sent some 100 medics, and the African Union has pledged to send around 100 more, while a range of other countries have pitched in with smaller teams and other aid.

WHO and the Doctors Without Borders charity meanwhile each count around 200 international medical experts on the ground in the region, while WHO has 300 more on standby.

- 2,000 orphans in Liberia -

While welcoming the push in Sierra Leone, Chan said Friday the situation in neighbouring Liberia was especially desperate, without a single bed left to treat Ebola patients.

The UN vowed its peacekeepers, who have been there since the country emerged from 14 years of civil war in 2003, would "stay the course."

The UN mission would "help the people of Liberia and its neighbours to get through this terrible crisis," UN peacekeeping chief Herve Ladsous told AFP late Thursday.

The UN children's agency warned Friday that the outbreak was already taking an especially dire toll on children in Liberia, which is expected to face an "exponential increase" in infections.

In addition to falling victim to Ebola themselves, many children are losing their caregivers, with some 2,000 orphaned in Liberia alone, said Sarah Crowe, head of UNICEF's crisis communication in Liberia.

Children who lose family members to Ebola meanwhile can face "deep stigma", and are often rejected by the community and forced to roam the streets without proper parental care, shelter or food, she said.

Around 20 percent of children who have lost family members to Ebola in Liberia are under the age of two, according to UNICEF.