Barack Obama, in a chilling assessment of international efforts to stem a deadly Ebola outbreak, said the world has not done enough to respond to a health crisis that poses a growing threat to regional and global security.

"There is still a significant gap between where we are and where we need to be," the US president said to a high-level United Nations meeting on Ebola.

The crisis in West Africa is the largest ever outbreak of Ebola, with more than 6,200 people believed to have been infected, almost half of whom have died. US health officials have warned that the number of infected people could explode to at least 1.4 million by mid-January, although they have also cautioned that the totals could peak well below that if efforts to control the outbreak are ramped up.

Margaret Chan, director of the World Health Organisation, addressed the Ebola meeting ahead of Mr Obama and warned that the outbreak will probably get worse before it gets better. The virus, she said, is "still running ahead, jumping over everything we put in place to slow it down".

Mr Obama has dispatched 3,000 US troops to Liberia to set up facilities and form training teams to help the Africans treat Ebola victims. Congress also approved the use of leftover Afghanistan war money to begin funding Obama's billion-dollar request to help fight the outbreak.

While Mr Obama touted US contributions, he warned other nations that the US does not have the capacity to fight the epidemic on its own.

"Everybody's got to move fast in order for us to make a difference," he said. "If we do, we'll save hundreds of thousands of lives."

UN secretary-general Ban Ki-moon also urged world leaders to "step up" efforts to fight Ebola.