According to NBC, citing an employee familiar with the company's response efforts, it "looks and smells like ransomware", an attack when criminals demand payments in return for a key to reopening encrypted files.
"The IT Network across Universal Health Services (UHS) facilities is currently offline, due to an IT security issue", the statement said.
Universal Healthcare Services explained that it is using back-up procedures, including offline documentation methods, while work is underway to restore online operations.
"Patient care continues to be delivered safely and effectively. No patient or employee data appears to have been accessed, copied or misused", the statement said.
Two nurses at Universal Health Services told NBC that the attack started over the weekend when their computers slowed and then completely went down on early Sunday morning.
It is one of the largest medical cyberattacks in the United States, according to media reports.