The Commission is requesting the providers of these services to provide more information on how they have diligently identified any systemic risks concerning the App Store and Google Play", the EU executive said in a statement.
The two firms were given a Jan. 15 deadline to reply.
They are part of a group of over a dozen of the world's biggest tech companies facing unprecedented legal scrutiny since the DSA came into force this year, including sweeping new obligations to tackle illegal content and online security risks.
The EU's list of questions also concerns transparency-related issues linked to recommender systems and online advertisements, the commission said, adding that potential next steps include the opening of formal proceedings.