Sierra Leone began a quarantine of more than one million people Thursday in the largest open-ended lockdown in the Ebola outbreak, as world leaders met to discuss the crisis at the United Nations.

The northern districts of Port Loko and Bombali have been closed off indefinitely along with the southern district of Moyamba -- effectively sealing in around 1.2 million people.

With the eastern districts of Kenema and Kailahun already under quarantine, more than a third of the population of six million -- in five of the nation's 14 districts -- now finds itself unable to move freely.

The president said 12 of the county's 149 tribal chiefdoms -- much smaller administrative areas than districts -- were also to be placed in quarantine, although the total population in these areas was not immediately clear.

"The isolation of districts and chiefdoms will definitely pose great difficulty but the lives of everyone and the survival of our country takes precedence over these difficulties," President Ernest Bai Koroma told the nation in a televised address late Wednesday.

The deadliest Ebola epidemic on record has infected more than 6,200 people in west Africa and killed nearly half of them, according to the World Health Organization's latest figures.

The virus can fell its victims within days, causing rampant fever, severe muscle pain, vomiting, diarrhoea and -- in many cases -- unstoppable internal and external bleeding.

In Sierra Leone, Ebola has infected 1,940 people, killing 593, by the WHO count, but the UN agency has warned the number of cases across the region could explode in the coming months without an urgent response.

- 'Horrific suffering' -

World leaders were due to attend a meeting in New York on Ebola convened by UN Secretary-General Ban Ki-moon later Thursday, with Koroma and Liberia's President Ellen Johnson Sirleaf connected by video link.

The meeting -- part of the United Nations General Assembly -- will hear from US President Barack Obama and world leaders are expected to pledge help for efforts to try to contain the spread of the virus.

Obama, who is sending 3,000 troops to west Africa to help health workers battle the contagion, urged other countries Wednesday to get behind a broader international effort.

"As we speak, America is deploying our doctors and scientists -- supported by our military -- to help contain the outbreak of Ebola and pursue new treatments," Obama told the 193-member assembly.

"But we need a broader effort to stop a disease that could kill hundreds of thousands, inflict horrific suffering, destabilise economies and move rapidly across borders."

The WHO warned Tuesday that without quicker prevention efforts, hundreds of thousands could be infected with Ebola by the end of the year.

The US Centers for Disease Control estimated that cases in Liberia and Sierra Leone could rocket to 1.4 million by January -- in a worst-case scenario based on data obtained before the world ramped up its response.

- Ending the outbreak -

Sierra Leone has revealed that around 100 bodies and 200 patients were collected from homes during a nationwide three-day lockdown and house-to-house information campaign which ended on Sunday.

Koroma said in his televised address the temporary curfew had prompted the new quarantine -- which is expected to remain in place until the crisis is under control.

The lockdown was "line with our people's avowed commitment to support extra measures to end the Ebola outbreak", Koroma said.

Residents contacted by AFP in the affected districts were divided over whether the quarantine was a positive development, although most agreed it had come as a shock.

"The community is in a sombre mood, as no one expected this development so soon," said Momodu Barrie, a resident of Makeni, the main town in Bombali and home town of the president.

"It caught people with their pants down but the red lights were already flashing, with the high toll of deaths occuring over the past few weeks."

In Port Loko, ferry operator Brima Sillah said he had accepted fairs to take people around the coast to Freetown.

"I have used up the money -- how can I find a way to give them it back?" he said.

The quarantine has also caught out people from the affected districts who were in Freetown when it was imposed.

"I left Port Loko only yesterday and there were no signs that this was going to happen," said Lamarana Sowe.

"I was shocked when I went to the motor park this morning to board a vehicle for Lunsar and found no transport available. Now I cannot go back."

The WHO said on Thursday 6,263 people had been infected since the virus first emerged in southern Guinea in December, and that 2,917 had died.

Liberia, the worst-hit nation with 1,677 deaths, announced in a statement from the presidency on Wednesday that China had pledged $1 million to help it fight the epidemic.