New York - New York health authorities were looking for possible new cases of swine flu at a second high school after 28 cases were confirmed at a nearby school, news reports said Tuesday.

The New York City department of health was investigating whether students at Public School 177 might have contracted the flu after it had confirmed that 28 students at the Saint Francis High School fell ill of the swine flu.

Mayor Michael Bloomberg said Monday there were 17 suspected and 28 confirmed cases at the Saint Francis school while testing was being carried out on more than 100 other students at the same school. Bloomberg said city authorities were focusing on Saint Francis, the only cluster of swine flu illness in the city.

The flu breakout at Saint Francis was traced to students who had spent the spring recess last month in Mexico, where an influenza-type illness has killed 149 people. The school remained closed for a second day on Tuesday so cleaning crews can sanitize the building.

City authorities said, however, that the confirmed flu cases were mild and required no hospitalization.

The World Health Organization in Geneva on Monday raised the flu threat to level four, two steps below a full blown pandemic, after new cases of swine flu were discovered in more countries around the world. WHO said level four means the disease is spreach through sustained human-to-human transmission and called for mitigating measures to prevent new cases.