Ending the deadliest Ebola outbreak in history is a difficult task, but it is "within our reach", the UN's new mission chief for the disease said, warning that the world has no choice but to beat back the infection.

"This is a global crisis. We definitely have a difficult time ahead of us, but we can achieve it," Ismail Ould Cheikh Ahmed, the new head of the UN Mission for Ebola Emergency Response, said on arrival in Ghana on Saturday.

"We have no plan B, we have to get rid of this virus. This is within our reach, but we should not be complacent," said Ahmed, who had arrived in Accra to take over from Anthony Banbury.

The UN mission, based in the Ghanaian capital, is leading international efforts in the battle against Ebola.

According to the latest World Health Organization figures, there are more than 20,200 confirmed, probable or suspected cases of Ebola and just over 7,900 reported deaths. The three worst-affected countries are Sierra Leone, Liberia and Guinea.

"We need to keep going until we don't have even one case, because even one case is too many," he said. "The work ahead remains very hard but we really have no other choice," the Ebola mission chief added.

Ahmed will be visiting Liberia and Sierra Leone this week, and Guinea shortly after, to reinforce the mission's strategic priorities and see firsthand the response to Ebola. He will be accompanied by UN Special Envoy on Ebola David Nabarro.