Germany's competition watchdog said on Tuesday that Facebook was abusing its dominant position to "limitlessly" harvest user data from outside websites and apps, allowing its advertisers to target customers with hyper-specific ads.

In a preliminary assessment, the Federal Cartel Office (FCO) said it had focused its probe on the US social media giant's use of third-party sites to track users' browsing behaviour, often without their knowledge.

"The authority holds the view that Facebook is abusing this dominant position by making the use of its social network conditional on its being allowed to limitlessly amass every kind of data generated by using third-party websites," the FCO said in a statement.

These third parties include Facebook-owned services such as WhatsApp and Instagram, but also sites and apps that are less obviously linked to Facebook, often through the "like" button at the bottom of a webpage.