UPDATE 2-WHO says flu pandemic remains at level 5 of 6

* Global pandemic alert remains at level 5 of 6, WHO says

* One in three people may eventually be infected by H1N1 flu

* Could take two years for strain to gain worldwide momentum

(Adds quotes on Europe paras 7-8, pandemic risks 12-14, 16)

By Laura MacInnis

GENEVA, May 7 (Reuters) - H1N1 flu is not yet spreading in a sustained way outside North America, so the global pandemic level remains at 5 out of 6, a senior World Health Organisation official said on Thursday.

"We remain at Phase 5. That is not changed," said Keiji Fukuda, acting WHO assistant director-general. "We continue to see human to human transmission, community-level transmission, primarily in North America. We are not seeing it yet anywhere else," he told a news conference.

WHO Director-General Margaret Chan last week raised the alert level to 5 -- indicating a pandemic is imminent -- and is waiting for evidence the new strain has taken hold outside the Americas before declaring a pandemic is under way.

A move to Phase 6 would speed up efforts already under way to produce and distribute antiviral drugs and vaccines.