Mayor Michael Bloomberg on Tuesday said there are now four confirmed cases of swine flu at Rikers Island, the city's main jail.

The mayor says authorities can't evacuate the jail, but steps are being taken to try to prevent the illness from spreading.

"We are watching it very carefully," Bloomberg said. "It is, in some senses, easier to control because obviously the prisoners can't leave. On the other hand, it is also a confined area where we really don't have the choice of moving people out and asking them to stay home."

The Correction Officers Benevolent Association on Tuesday accused the city of underreporting the H1N1 cases on Rikers Island and said thousands of lives could be at risk if the city doesn't take appropriate measures at the jail.

AP wire services were used in this report.