BRUSSELS (Reuters) - The World Health Organization (news - web sites) is bracing for another SARS (news - web sites) outbreak this winter and is setting up a rapid reaction center at its Geneva headquarters, the head of the United Nations (news - web sites) agency said on Wednesday.

Dr Lee Jong-Wook, speaking at the European Parliament, told reporters it was impossible to be sure how the killer virus would behave in coming months.

"Our working assumption is that SARS will come back," Lee said.

"In the coming winter, if SARS is mixed with the common cold and flu this will cause a lot of problems."

Severe Acute Respiratory Syndrome (SARS) killed more than 700 people and infected over 8,400 this year after first appearing in southern China last November.

The WHO declared the outbreak over in July. There has been one isolated case in Singapore since then.

Lee said that given the disease had not completed a one year cycle it was not known how it would behave in the winter months.

Developing better diagnostic tests was critical to handling any further outbreak, he added.

Lee, who took up his post in July, added the WHO was constructing a rapid reaction center at its Geneva headquarters to manage any future outbreak.

The WHO has also sent scientists to China to research animals which may pass the disease to humans.

Lee warned last month that the world would face many more SARS-like diseases in the 21st century.

WHO officials this week called for tighter controls at laboratories where specimens of the virus are stored and studied, fearing they could be the source of another outbreak.

A Singaporean medical student contracted SARS after a laboratory accident in September. He has since recovered.