Apple on Friday rsponded to Spotify's claims that the U.S. tech giant had hampered competition in music streaming by rejecting an update to Spotify's iPhone app. The two companies have gone head to head in the battle for music streaming customers since Apple Music was launched last year.

Spotify has argued that the 30 percent cut Apple takes of subscriptions in its App Store give its own service an unfair advantage. Spotify General Counsel Horacio Gutierrez reiterated those concerns in a letter to Apple first reported on Thursday as he protested the rejection of the latest version of the Spotify app.

Apple General Counsel Bruce Sewell countered that the company deserves a cut of transactions in the App Store for its work operating the marketplace.

"We understand that you want special treatment and protections from competition, but we simply will not do that because we firmly adhere to the principle of treating all developers fairly and equitably," Sewell wrote.

Gutierrez claimed Apple's rejection of Spotify's app raised "serious concerns" under competition law in the United States and Europe and the move was causing "grave harm to Spotify and its customers," according to Recode.

Spotify is the world's biggest paid music streaming service with about 30 million paying users in 59 markets while Apple Music has some 13 million.

Spotify have sought to sidestep Apple's App Store cut by encouraging consumers to sign up for their services online. Apple forbids developers from promoting alternative payment methods within their apps.

Last May, Spotify submitted a version of its app that removed the in-app purchase feature, which triggers Apple's cut, and included an account sign-up feature that violated Apple's rules, Sewell wrote. Apple rejected the app and asked Spotify to submit again, but the new version had the same problems, Sewell said.

Google Music and YouTube also compete with Spotify and Apple Music to attract users prepared to pay for music, as does Pandora Media and rapper Jay Z's Tidal.

Amazon.com is also preparing a standalone streaming service.