LONDON (AP) -- Ireland's health service shut down its IT systems on Friday after being targeted in what it called a "significant ransomware attack."

The Health Service Executive said the move was a precaution, and appointments for coronavirus vaccinations were not affected.

"We've taken a precautionary measure to shut down a lot of our major systems to protect them," chief executive Paul Reid told broadcaster RTE.

"We are at the very early stages of fully understanding the threats, the impact and trying to contain it."

It was unclear how wide the disruption to the health system was. Dublin's Rotunda maternity hospital said it was canceling most routine appointments due to the IT issues, calling the situation a "critical emergency."

Ransomware attacks are typically carried out by criminal hackers who scramble data, paralyzing victims' networks, and demand a large payment to decrypt it.

Health care systems have been a target before. In 2017, Britain's National Health Service was hit in a cyberextortion attack, forcing hospitals to close wards and emergency rooms and turn away patients.

In the U.S., the nation's largest fuel pipeline was hit with a ransomware attack a week ago. The disruption of the Colonial Pipeline caused long lines at gas stations due to distribution problems and panic-buying, draining supplies at thousands of gas stations. It restarted operations on Wednesday.

The attack highlighted concerns about the vulnerability of critical infrastructure to hacking groups and criminals.