WRAPUP 10-WHO declares first 21st century flu pandemic

* Virus is causing moderate disease

* Phase 6 means spread is inevitable

* WHO urges countries to finish seasonal vaccine production (Adds comments from U.S. officials, genetics study)

By Jonathan Lynn

GENEVA, June 11 (Reuters) - The World Health Organization declared an influenza pandemic on Thursday and advised governments to prepare for a long-term battle against an unstoppable new flu virus.

The United Nations agency raised its pandemic flu alert to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way.

"With today's announcement, WHO moves from an emergency to a longer-term response. Based on past experience, this pandemic will be with us for some months, if not years, to come," WHO Director-General Dr. Margaret Chan said in a letter to staff, a copy of which was obtained by Reuters.

People aged 30-50, pregnant women or people suffering from chronic conditions such as asthma, diabetes or obesity are at highest risk, Chan told a news conference.

Countries from Australia to Chile and the United States are reporting the new swine flu virus is "crowding out" seasonal flu, becoming the predominant influenza strain, she said.