A cyberattack on a US hospital group has forced the closure of urgent care and other critical health services at its facilities across at least four states, officials said Friday.

Prospect Medical Holdings, which operates 16 hospitals across California, Connecticut, Pennsylvania and Rhode Island, suffered a "data security incident," a spokeswoman said in a statement to AFP.

"Upon learning of this, we took our systems offline to protect them and launched an investigation with the help of third-party cybersecurity specialists," she said.

The group's Waterbury Hospital in Connecticut said the computer outage was affecting all inpatient and outpatient operations, with staff using "paper records, until this is resolved."

The Eastern Connecticut Health Network said its urgent care center was closed, and elective surgeries canceled, until further notice.

The incident appeared to be a ransomware attack, but was still under investigation, the spokeswoman said.

Ransomware attacks typically access vulnerable computer systems and encrypt or steal data, before sending a ransom note demanding payment in exchange for decrypting the data or not releasing it publicly.

Healthcare groups are frequently targeted.

"While our investigation continues, we are focused on addressing the pressing needs of our patients as we work diligently to return to normal operations as quickly as possible," said the statement.