Facebook, Instagram and WhatsApp went offline for users across the globe, the social media giant said on Monday, as it worked on restoring its services.

Reuters could not immediately confirm the issue affecting the services, but the error message on the webpage of Facebook Inc suggested a problem with Domain Name System (DNS).

DNS allows web addresses to take users to their destinations. A similar outage at cloud company Akamai Technologies Inc took down multiple websites in July.

Security experts tracking the situation said the outage could have been triggered by a configuration error, which could be the result of an internal mistake, though sabotage by an insider would be theoretically possible.

An outside hack was viewed as less likely. A massive denial-of-service attack that could overwhelm one of the world’s most popular sites, on the other hand, would require either coordination among powerful criminal groups or a very innovative technique.

Facebook acknowledged users were having trouble accessing its apps but did not provide any specifics about the nature of the problem or how many were affected by the outage. “We’re working to get things back to normal as quickly as possible, and we apologise for any inconvenience,” Facebook said.

Shares of Facebook, which has nearly 2 billion daily active users, fell 5.5 per cent in afternoon trading on Monday, inching towards its worst day in nearly a year.

Downdetector — which only tracks outages by collating status reports from a series of sources, including user-submitted errors on its platform — showed there were more than 50,000 incidents of people reporting issues with Facebook and Instagram. The outage might be affecting a larger number of users.

Meanwhile, the social-media giant’s instant messaging platform WhatsApp was also down for over 35,000 users, while Messenger was down for nearly 9,800 users.

Facebook has experienced similar widespread outages with its suite of apps this year in March and July.

Several users using their Facebook credentials to log in to third-party apps such as Pokemon Go and Match Masters were also facing issues. “If your game isn’t running as usual please note that there’s been an issue with Facebook login servers.