EU antitrust regulators launched proceedings on Thursday to ensure that Apple complies with landmark rules requiring it to open up its closed ecosystem to rivals or risk a possible hefty fine.

Under the so-called specification proceedings, the European Commission will spell out what Apple has to do to abide by the Digital Markets Act (DMA), which came into effect last year.

"Today is the first time we use specification proceedings under the DMA to guide Apple towards effective compliance with its interoperability obligations through constructive dialogue," EU antitrust chief Margrethe Vestager said in a statement.

The EU competition enforcer said the first proceeding targets iOS connectivity features and functionalities for smartwatches, headphones, virtual reality headsets and other internet-connected devices.

"The Commission intends to specify how Apple will provide effective interoperability with functionalities such as notifications, device pairing and connectivity," it said.

The second proceeding concerns how Apple addresses interoperability requests submitted by developers and third parties for iOS and iPadOS. Both processes will be wrapped up within six months.

"It is crucial that the request process is transparent, timely, and fair so that all developers have an effective and predictable path to interoperability and are enabled to innovate," the Commission said.

Apple said it will continue to work constructively with the Commission but also warned of the risks.

"Undermining the protections we've built over time would put European consumers at risk, giving bad actors more ways to access their devices and data," it said in a statement.