A day after Facebook CEO Mark Zuckerberg accused Apple of abusing its control over the iPhone app ecosystem, Apple CEO Tim Cook fired back.

In a speech on Thursday, Cook didn’t mention Facebook by name, but made clear that Apple would move ahead with modifications to the iPhone’s software that are aimed at blocking online advertisers from tracking consumers. Zuckerberg, whose business is based on selling ads, opposes the changes.

“Users have asked for this feature for a long time,” Cook said at the Computers, Privacy, and Data Protection conference. “We have worked closely with developers to give them the time and resources to implement it. And we’re passionate about it because we think it has the great potential to make things better for everybody.”  

Apple first announced the feature, called App Tracking Transparency, in June at its Worldwide Developers Conference. Once enabled, any app that wants to track an iPhone user’s location, activities, contacts, and other data first would have to ask the user for permission. Amid complaints from Facebook and advertisers that few users would allow data collection, Apple delayed the feature’s debut from the end of 2020 to sometime later this year.

Apple now says the feature will arrive in the spring.

“Right now, users may not know whether the apps they use to pass the time, to check in with their friends, or to find a place to eat, may in fact be passing on information about the photos they’ve taken, the people in their contact list, or location data that reflects where they eat, sleep, or pray,” Cook explained in his speech. Then, using the acronym for Apple’s new feature, he added, “When ATT is in full effect, users will have a say over this kind of tracking.”

Facebook and marketers complain that the cost of protecting user privacy will be a huge drop in revenue from targeted ads that rely on the data. During a call with analysts on Wednesday, Zuckerberg said Apple shouldn’t have the power to cut off data tracking by apps. Facebook is also considering suing Apple for antitrust violations, tech news site The Information reported on Thursday.

“Apple has every incentive to use their dominant platform position to interfere with how our apps and other apps work, which they regularly do to preference their own,” Zuckerberg said. “Apple may say they’re doing this to help people, but the moves clearly track their competitive interests.”

Federal antitrust regulators and lawmakers have both companies in their sights. In December, the Federal Trade Commission and attorneys general from 46 states filed an antitrust lawsuit against Facebook for thwarting competition by buying potential competitors like WhatsApp and Instagram. Meanwhile, House Judiciary Committee investigators have targeted Apple over its control of the iPhone app store, and Epic Games has sued Apple for alleged antitrust violations.

Over the past few years, Apple has already taken steps to protect the privacy of iPhone users, including allowing users to limit location tracking by apps and adding a “privacy nutrition label,” requiring apps to disclose the kinds of data they collect.

In his speech, Cook also highlighted another criticism of Facebook and other social networks—that they helped spread dangerous misinformation, again without naming specific companies.

“At a moment of rampant disinformation and conspiracy theories juiced by algorithms, we can no longer turn a blind eye to a theory of technology that says all engagement is good engagement—the longer the better—and all with the goal of collecting as much data as possible,” he said. “It is long past time to stop pretending that this approach doesn’t come with a cost—of polarization, of lost trust, and, yes, of violence.”