Facebook (FB.O) and Microsoft’s (MSFT.O) grievances over how their gaming apps appear on Apple’s (AAPL.O) App Store may feed into an EU investigation into the iPhone maker’s business as EU antitrust regulators said such concerns are on their radar.

The European Commission in June opened four probes into Apple, three of which are into its App Store and its restrictive rules, including requirements that app developers use its own in-app purchasing system.

U.S. social media giant Facebook and Microsoft are the latest companies to voice concerns about the rules, which have drawn criticism from app developers who say they create an uneven playing field to compete with the iPhone maker.

Asked about Facebook and Microsoft’s issues with Apple, Commission spokeswoman Arianna Podesta said in a statement: “The Commission is aware of these concerns regarding Apple’s App Store rules.”

She did not provide details.

Apple dismissed criticism of its App Store rules, saying the same set of rules applies to all apps to protect customers and provide a fair and level playing field for developers.

Sony’s PlayStation Remote and Valve’s SteamLink are allowed on the App Store, it said, and developers can reach users via its web browser Safari, where they can also build stores and services.

Facebook last week said its gaming app was only available on Apple’s App Store as a streaming service and that users will not be able to play games.

Facebook Chief Operating Officer Sheryl Sandberg said the company had to remove gameplay functionality entirely to secure Apple’s approval of its Facebook Gaming app.

Microsoft, which has a game-streaming service called Project xCloud said: “Apple stands alone as the only general purpose platform to deny consumers from cloud gaming and game subscription services like Xbox Game Pass.”

“It consistently treats gaming apps differently, applying more lenient rules to non-gaming apps even when they include interactive content,” it added in an emailed statement.