By Tom Miles

GENEVA (Reuters) - Two people have fallen ill with Ebola in Guinea, the World Health Organization said on Friday, dashing hopes of an imminent end to the worst recorded outbreak of the disease after a two-week spell without any new cases across West Africa.

The new cases mean the epidemic, which began when a 2-year-old boy who fell ill in a remote Guinean village on December 26, 2013, risks dragging on into a third year and into 2016.

The outbreak has already killed 11,298 people out of almost 28,500 known cases in Guinea and neighbouring Liberia and Sierra Leone.

Liberia was declared free of Ebola transmission on Sept. 3 after 42 days with no new cases, and Sierra Leone is halfway to the same goal. But for Guinea, the end is not yet in sight.

"Guinea is not at the stage where they would be counting to zero cases. While you've got active confirmed cases you don't even consider that," said WHO spokeswoman Margaret Harris.

The 42-day countdown only starts once the last patient tests negative a second time, normally after a 48-hour gap following their first negative test, Harris said.

A spokesman for Guinea's anti-Ebola taskforce said one of the new patients, in Forecariah in western Guinea, had recovered and was discharged on Friday. The other, a 21-year-old man, who was not known to have had contact with any previous registered patients, remained in the Nongo treatment centre in Conakry.

"We suspect that he contracted the disease by another means, perhaps sexual, but we cannot be sure for the time being," said Fode Tass Sylla.

A study this week showed the semen of male survivors can harbour the virus for nine months, and the virus can also live on in other parts of the body.

Highlighting the lingering risks from the virus, a British nurse fell critically ill this week, 10 months after recovering from Ebola.

The treatment of Ebola has been transformed in the past few months by the success of a trial vaccine, which is now used to treat each new case and their contacts who may also be at risk from the highly contagious virus.

But Harris said that the trial comes to an end in mid-November, potentially putting a question mark over the use of the vaccine beyond that date.