By Foo Yun Chee

BRUSSELS (Reuters) -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.

It will specify how Apple will provide effective interoperability with functionalities such as notifications, device pairing and connectivity.

The second proceeding concerns how Apple addresses interoperability requests submitted by developers and third parties for iOS and iPadOS, with the company told ensure a transparent, timely, and fair process.

The Commission aims to wrap up both proceedings within six months.

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.

Apple risks fines of as much as 10% of its annual global turnover if it fails to comply with the DMA.

(Reporting by Foo Yun Chee;Editing by Sudip Kar-Gupta, Philippa Fletcher and Tomasz Janowski)