Four students from Texas have been confirmed carrying the H1N1 virus and are under “close supervision,” health authorities said Thursday.

They are the first cases to be discovered in Cambodia, following the announcement this month the spread of the virus, sometimes called swine flu, had become a pandemic.

The four students, including three girls, between the ages of 16 and 20, arrived in Cambodia June 19.

The four are under the supervision of health officials, who are preventing them from travel while they monitor the situation, Health Minister Mam Bunheng told reporters during a joint press conference with the World Health Organization.

Cambodia’s first case of the virus, in a 16-year-old student, was announced on Wednesday.

The virus has spread to more than 100 countries, including China and Thailand, and has killed at least 250 people, while infecting as many as 56,000.

However, Mike O’Leary, chief of Cambodia’s WHO office, said there was no cause for alarm.