The world can stamp out the Ebola outbreak in West Africa by the end of the year but months of tough work remain, the outgoing chief of the United Nations' anti-Ebola mission said on Friday.

The outbreak, first identified in Guinea's remote southeast in early 2014, has struck six West African nations, with Guinea, Sierra Leone and Liberia bearing the brunt of the 20,000 infections and nearly 8,000 dead.

Faced with criticism the world was not doing enough, U.N. Secretary General Ban Ki-moon set up the U.N. Ebola Emergency Response (UNMEER) mission in September to coordinate global efforts.

"We have not come anywhere close to ending the crisis. We’ve done a lot in 90 days in a very successful response but we have a long and difficult way to go," Anthony Banbury told reporters in Accra, where the U.N. mission is based.

"It's going to go on for not just weeks but some months more. But I believe we will do it in 2015 and we’re going to do it by working very closely not just with governments of the countries but the communities," he said.

Banbury will be replaced by veteran humanitarian official Ould Cheikh Ahmed of Mauritania on Saturday.

A spike of cases in Sierra Leone meant UNMEER missed its target of ensuring that by early December 70 percent of all Ebola patients were being treated in isolation units and 70 percent of all those who died from Ebola were buried properly.

Banbury said there were now enough functioning treatment centers in the region.

The target of 100 percent safe burials by end of January 2015 was on track now that there were some 254 safe burial teams operating in the affected countries.

Six other countries, including Nigeria, Senegal, Mali, the United States, Spain and Britain, have reported cases imported from the worst affected countries.

Last week, a nurse was diagnosed with the virus in Britain upon her return from Sierra Leone. She is being treated with blood plasma from a survivor of the virus and an experimental antiviral drug, according to the London hospital treating her.

(Reporting by Kwasi Kpodo; Editing by David Lewis and Robin Pomeroy)