The swine flu pandemic has grown "unstoppable" and all nations will need access to vaccines, a WHO official said on Monday, as 12 new deaths were reported and a study raised fresh concerns.

Britain, Brazil, Colombia, Mexico, the Philippines and Thailand all reported deaths on Monday, while Saudi Arabia shut an international school after 20 students were diagnosed with the H1N1 virus.

As the death toll increased, the World Health Organization official said a swine flu vaccine should be available as early as September and all countries would need to be able to protect themselves.