More than 1,000 Russian schools have closed amid a worsening flu epidemic that has already afflicted one in five Russians, state news agency TASS reported.

The number of flu cases nationwide has risen about 25 percent since last week, the news agency reported, citing the Influenza Research Institute in St. Petersburg, and in one case, an 11-year-old girl in Yekaterinburg died.

The Siberian city of Tyumen has become the latest place to close its schools, with the local school board announcing an 11-day break on its website Thursday.

The closure is under way "to prevent an increase in incidences of acute respiratory viral infection and influenza among students," the site said, after the region's health watchdog reported more than 15,000 cases in the past week.

Moscow saw an 11.8 percent increase in flu cases last week, below the 16.9 percent threshold to be considered an epidemic, health watchdog Rospotrebnadzor said in a statement. Schools in the area remain open.

The watchdog urged infected Muscovites to not self-medicate and "immediately consult your doctor for medical advice." It also instructed schools to immediately isolate children with symptoms of infection.

Last week President Vladimir Putin advised high-ranking officials not to go to work if they were feeling ill, after Prime Minister Dmitry Medvedev reportedly came down with the flu.