Germany's anti-cartel watchdog said Monday it is examining whether a deal between Amazon subsidiary Audible.com and Apple to distribute digital audio books flouted rules.

The two US giants have a long-standing agreement in which audio books offered for sale on Apple's iTunes store are sold through Audible.

But the Federal Cartel Authority is now taking a closer look at the agreement after the Association of German Publishers and Booksellers questioned the exclusive deal.

"We have to ensure that publishers of audio books have sufficient alternatives to distribute their digital audio books," said Andreas Mundt, who heads the Federal Cartel Authority.

Audible is a leading distributor of digital audio books in Germany, offering its services through its own Audible.de website as well as through its parent company's Amazon.de.

Both Amazon and Apple have been regular targets of anti-trust probes given their leading positions in online retail.

In June, a US appeals court upheld a 2013 ruling that Apple led an illegal conspiracy to fix prices of e-books in violation of anti-trust laws.

In the same month, the EU's anti-trust watchdog opened a formal investigation into Amazon's e-book distribution.