Facebook said it would appeal against an order by a German privacy regulator on Tuesday to stop collecting and storing data of German users of its messaging app WhatsApp and to delete all data that has already been forwarded to it.

The Hamburg Commissioner for Data Protection and Freedom of Information said Facebook was infringing data protection law and had not obtained effective approval from WhatsApp's 35 million users in Germany.

"After the acquisition of WhatsApp by Facebook two years ago, both parties have publicly assured that data will not be shared between them," commissioner Johannes Caspar said in a statement.

Facebook, the world's biggest social network, bought WhatsApp for $19 billion in cash and stock in an effort to reach a younger audience.

"The fact that this is now happening is not only a misleading of their users and the public, but also constitutes an infringement of national data protection law," Caspar added.

Facebook has its German headquarters in Hamburg and therefore falls under Caspar's jurisdiction.

"We will appeal this order and we will work with the Hamburg DPA in an effort to address their questions and resolve any concerns," it said.

The data watchdog said Facebook and WhatsApp were independent companies that should process their users' data based on their own terms and conditions and data privacy policies.

EU and US regulators have said they would scrutinise changes to privacy settings that WhatsApp made in August.

Italy's data privacy watchdog said on Tuesday it had also opened a probe and had asked WhatsApp to explain what information it planned to share with Facebook and what was being done to explain to users how their data might be used.