* WHO chief says too early to relax about flu outbreaks

* Margaret Chan calls for fast sharing of H1N1 samples

* Parts of Southeast Asia seen at particular threat

(Adds Ecuador in paragraph 16)

By Stephanie Nebehay and Laura MacInnis

GENEVA, May 15 (Reuters) - The World Health Organization warned on Friday against a false sense of security from waning and apparently mild outbreaks of H1N1 flu, saying the worst may not be over.

WHO Director-General Margaret Chan said there remained "great uncertainty" about the new strain that continues to spread and could pose particular threats in Southeast Asia.

But she could not say whether or when the United Nations agency might raise its pandemic alert to the highest level from the current 5 on a scale of 6. The trigger would be if sustained spread was confirmed in communities outside of North America.

"Actually, I am asking myself that question every day," Chan said in response to a question from Argentina's delegation.

"We are meeting at a time of crisis that could have global implications," she warned the intergovernmental meeting on pandemic influenza preparedness at WHO's Geneva headquarters. "This is a virus so evasive that it can quietly and stealthily move into your country without you even realising it."

The two-day meeting is tackling the sensitive issue of virus sharing in exchange for access to vaccines derived from them.

At the height of fears about bird flu, Indonesia had refused to share H5N1 virus samples without guarantees the vaccines would be provided to poorer countries at an affordable price.

The negotiations, begun in November 2007, have taken on fresh urgency with the emergence of the H1N1 virus.

If negotiators reach a draft agreement, it would be brought to the WHO's annual assembly of health ministers, who meet in Geneva next week, for possible adoption.

GlaxoSmithKline, Sanofi-Aventis, Novartis, Baxter International and other drug makers are awaiting WHO guidance about whether to start mass-producing vaccines to fight H1N1, which may force them to cut production of seasonal flu shots.

Glaxo said Friday it had received orders from several governments, including Britain, France, Belgium and Finland, looking to stockpile a pandemic vaccine against the new virus.

Chan said she would make a recommendation soon about the appropriate balance between making the two types of injections.

"We are moving on two tracks to ensure some security for seasonal vaccine and at the same time kick-starting early scientific work for pandemic vaccine," she told the session.

SHARING SAMPLES

The WHO chief commended countries with H1N1 infections for their "timely sharing of samples for risk assessment and making seed vaccine," saying the starting point for larger production of injections could be ready by the end of this month.

Participants are seeking to reach agreement on standards for transparency, trust, and sovereignty related to sample sharing.

According to the latest WHO count, around 7,520 people in 34 countries have been infected with the strain that is a genetic mixture of swine, bird and human viruses. Belgium was the latest addition to the official tally. Ecuador also reported its first confirmed case.

Mexico has officially recorded 60 deaths from the virus that has also killed three people in the United States, one person in Canada and one in Costa Rica.

Most patients infected with the flu, which spreads like seasonal flu through sneezes, coughs and air droplets, have experienced mild symptoms and some appear to be asymptomatic.

But Chan stressed that important risks remain. "It is important that countries where cases are imported do not fall into a false sense of security. So make sure cases returning from travel do not spread in your community."

She said the WHO is closely watching parts of Southeast Asia that saw large outbreaks of H5N1 avian flu -- a virus that can be deadly when it passes from birds to humans, but has not spread easily between people to date.

A mixture of H5N1 and H1N1 viruses could have a big impact, she said, while stressing: "I am not saying it will happen." (Additional reporting by Ben Deighton in London; editing by Myra MacDonald)

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