The problems began last week after hackers gained access to Change Healthcare's information technology systems and has led to disruptions at pharmacies across the United States.

Change Healthcare and UnitedHealth did not immediately respond to a request for comment. Blackcat, also known as "ALPHV," did not immediately respond when asked whether it was responsible.

CISA, the U.S. cyber watchdog agency, and the FBI also did not immediately respond to emails seeking comment.

Blackcat is one of the most notorious of the internet's many ransomware gangs - groups of cybercriminals who encrypt data to hold it hostage with the aim of securing massive cryptocurrency payouts. It has previous struck major businesses including MGM Resorts International and Caesars Entertainment.

A number of pharmacy chains, including CVS Health and Walgreens, have said the outage at Change Healthcare was having knock-on effects on their businesses.

The American Pharmacists Association (APhA) said on Friday many pharmacies across the nation could not transmit insurance claims for their patients following the hack.

It said pharmacies were reporting "significant backlogs of prescriptions," which they were unable to process.

(Reporting by Raphael Satter and Christopher Bing in Washington; Additional reporting by Pratik Jain in Bengaluru; Editing by Sriraj Kalluvila, Shilpi Majumdar and Bill Berkrot)

By Raphael Satter and Christopher Bing