Washington, Oct 29 (IANS): American federal agents have issued a warning that a major ransomware assault was underway against US hospitals, threatening the country's healthcare system.

The advisory on Wednesday was issued by a joint federal task force that includes the Federal Bureau of Investigation (FBI), the Department of Health and Human Services (HHS), and the Cybersecurity and Infrastructure Security Agency (CISA), reports USA Today.

According to the agencies, at least five hospitals were attacked this week.

"(The) CISA, FBI, and HHS have credible information of an increased and imminent cybercrime threat to US hospitals and healthcare providers.

"CISA, FBI, and HHS are sharing this information to provide warning to healthcare providers to ensure that they take timely and reasonable precautions to protect their networks from these threats," the advisory said.

Independent security experts said the assault could potentially impact hundreds more institutions over the next few weeks.

"We are experiencing the most significant cyber security threat we've ever seen in the US," USA Today quoted Charles Carmakal, Chief Technical Officer of the cybersecurity firm Mandiant, as saying on Wednesday.

Alex Holden, CEO of Hold Security, said the latest assault was "unprecedented in magnitude for the US".

Ransomware is a type of malware which encrypts a victim's files.

The attacker typically demands a payment from victims to restore access to the blocked data.

The US has witnessed a sharp rise in ransomware cases over the past months, and hospitals have been particularly vulnerable amid the Covid-19 pandemic.

In the third quarter of this year, there were 313 such attacks in the country.

Globally, the percentage of the ransomware-affected healthcare organizations increased from 2.3 per cent in the second quarter to 4 per cent in the third.