Dr Bruce Aylward, WHO's assistant director-general, said on Tuesday that the death rate was higher than the official 50 percent rate and that "a lot more people will die" if the West Africa outbreak was not stopped.
"What we're finding is 70 percent mortality," Aylward said, adding that he had a "working forecast" of 5,000 to 10,000 new cases a week by December to guide the international response. "It's been running at about a thousand cases a week now for about three to four weeks. The labs sometimes can't keep up with the amount of specimens they're getting."
The announcement comes as the Texas Department of Health Services announced a second health worker at the Texas Health Presbyterian Hospital has tested positive for Ebola.
WHO figures released on Tuesday show 8,914 confirmed cases and a total of 4,447 people dead. However, WHO has said several times that the tallies are unreliable due to difficult recording conditions and workload.
Hardest hit countries
Sierra Leone, Guinea and Liberia have been hardest hit.
"There are this many cases that we're aware of, this many deaths that have been reported to us, but that doesn't mean you divide one by the other and get how many this disease kills," Aylward said. "To get that number, you need to take a bunch of people, follow them right through the course of their disease, and understand how many survive. "That subset of people, who we know were sick, and we know their final outcome, what we're finding is 70 percent mortality. "It's almost the exact same number across the three countries," he said.
Aylward said WHO needs to isolate 70 percent of cases within two months to contain the outbreak.
"Every time you isolate another patient, every time you have a safe burial, you're taking some of the heat out if this outbreak," he said. "But this is Ebola. This is a horrible, unforgiving disease. You've got to get to zero."
Death in Germany
Aylward's comments came on a day a UN medical worker infected with Ebola in Liberia died in Germany. The St Georg hospital in Leipzig said on Tuesday that the 56-year-old man, whose name has not been released, died overnight of the infection.
The man tested positive for Ebola on October 6, prompting Liberia's UN peacekeeping mission to place 41 other staff members under "close medical observation". Also on Tuesday, a UN official gave warning that the world was failing to gain the upper hand against the deadly outbreak.
"Ebola got a head start on us," Anthony Banbury, the British head of the UN Mission for Ebola Emergency Response, said.
Addressing the UN Security Council in New York by remote link from UNMEER headquarters in Accra, Banbury said: "It is far ahead of us, it is running faster than us, and it is winning the race.
"If Ebola wins, we the peoples of the United Nations lose so very much. "We either stop Ebola now or we face an entirely unprecedented situation for which we do not have a plan."