A ransomware attack is disrupting pharmacies and hospitals nationwide, leaving patients with problems filling prescriptions or seeking medical treatment.

On Thursday, UnitedHealth Group accused a notorious ransomware gang known as Black Cat, or AlphV, of hacking health care payment systems across the country.

Last week, the top health insurance company disclosed that its subsidiary, Optum, was impacted by a "cybersecurity issue," leading to its digital health care payment platform, known as Change Healthcare, being knocked offline.

As a result, hospitals, pharmacies and other health care providers have either been unable to access the popular payment platform, or have purposefully shut off connections to its network to prevent the hackers from gaining further access.

UnitedHealth says that as of Monday it estimated that more than 90% of 70,000 pharmacies in the U.S. have had to change how they process electronic claims as a result of the outage.