Girls wearing masks play outside to prevent from being infected by the swine flu virus in the Mexico City, capital of Mexico, on April 25, 2009. [Xinhua]

The World Health Organization (WHO) on Saturday declared the swine flu outbreak in Mexico and the United States a "public health emergency of international concern" and urged countries to be alert.

Based on the advice of a WHO emergency committee, Director-general Margaret Chan "has determined that the current events constitute a public health emergency of international concern," the UN agency said in a statement.

The WHO chief is recommending, on the advice of the committee, that all countries intensify surveillance for unusual outbreaks of influenza-like illness and severe pneumonia, the statement added.

However, more information is needed before a decision could be made concerning the appropriateness of the current "phase 3" alert level of a pandemic risk, according to the advice of the committee, which is composed of international experts in a variety of disciplines.

The WHO's pandemic alert includes altogether six phases. "Phase3" means "no or very limited human-to-human transmission."

The emergency committee was established in compliance with the International Health Regulations (2005). It convened its first meeting on Saturday to assess the swine flu situation and advise the WHO chief on appropriate responses.

After reviewing available data on the current situation, committee members identified a number of gaps in knowledge about the clinical features, epidemiology, and virology of reported cases and the appropriate responses, the WHO statement said.

The committee also noted that answers to several specific questions were needed to facilitate its work.

(Xinhua News Agency April 26, 2009)