BRUSSELS (Reuters) - EU antitrust regulators are seeking more information on Apple's (AAPL.O) mobile payment system, the European Commission said on Wednesday, a sign that the enforcer is looking to close any loopholes and boost its case against the iPhone maker.

The EU competition watchdog last year accused Apple of restricting rivals' access to its tap-and-go technology, Near-Field Communication (NFC), used for mobile wallets, making it difficult for them to develop rival services on Apple devices.

"We can confirm the sending of requests for information," a commission spokesperson said, while declining to provide details.

Apple declined to comment.

Apple has previously pointed to PayPal's (PYPL.O) success on its iOS mobile operating system as an option for users as well as competition from Danish rival MobilePay, Sweden's Swish and Belgium's Payconiq.

Norwegian mobile payment app and complainant Vipps said, however, that alternatives to NFC are cumbersome and not competitive.

The commission's request for information to rivals and retailers is unusual as it comes three months after Apple defended itself at a Feb. 14 hearing.

The regulator, which can fine Apple up to 10% of its global turnover if found guilty of breaching antitrust rules, typically issues decisions after such hearings.