In a major move from a private company, Apple will place new software in its iPhones that will help locate child sexual abusers in the United States.

Apple unveiled plans to scan U.S. iPhones for images of child sexual abuse, drawing applause from child protection groups but raising concern among some security researchers that the system could be misused by governments looking to surveil their citizens.

Apple said its messaging app will use on-device machine learning to warn about sensitive content without making private communications readable by the company. The tool Apple calls "neuralMatch" will detect known images of child sexual abuse without decrypting people's messages. If it finds a match, the image will be reviewed by a human who can notify law enforcement if necessary.

But researchers say the tool could be put to other purposes such as government surveillance of dissidents or protesters. Matthew Green of Johns Hopkins, a top cryptography researcher, was concerned that it could be used to frame innocent people by sending them harmless but malicious images designed to appear as matches for child porn, fooling Apple’s algorithm and alerting law enforcement – essentially framing people.

"Researchers have been able to do this pretty easily," he said.

Tech companies including Microsoft, Google, Facebook and others have for years been sharing "hashlists" of known images of child sexual abuse. Apple has also been scanning user files stored in its iCloud service, which is not as securely encrypted as its messages, for such images.

The company has been under pressure from governments and law enforcement to allow for surveillance of encrypted data. Coming up with the security measures required Apple to perform a delicate balancing act between cracking down on the exploitation of children while keeping its high-profile commitment to protecting the privacy of its users.

Apple believes it pulled off that feat with technology that it developed in consultation with several prominent cryptographers, including Stanford University professor Dan Boneh, whose work in the field has won a Turing Award, often called technology’s version of the Nobel Prize. The computer scientist who more than a decade ago invented PhotoDNA, the technology used by law enforcement to identify child pornography online, acknowledged the potential for abuse of Apple's system but said it was far outweighed by the imperative of battling child sexual abuse.

"Is it possible? Of course. But is it something that I’m concerned about? No," said Hany Farid, a researcher at the University of California at Berkeley, who argues that plenty of other programs designed to secure devices from various threats haven't seen "this type of mission creep." For example, WhatsApp provides users with end-to-end encryption to protect their privacy, but employs a system for detecting malware and warning users not to click on harmful links.

Apple was one of the first major companies to embrace "end-to-end" encryption, in which messages are scrambled so that only their senders and recipients can read them. Law enforcement, however, has long pressured for access to that information in order to investigate crimes such as terrorism or child sexual exploitation.

"Apple’s expanded protection for children is a game-changer," John Clark, the president and CEO of the National Center for Missing and Exploited Children, said in a statement. "With so many people using Apple products, these new safety measures have lifesaving potential for children who are being enticed online and whose horrific images are being circulated in child sexual abuse material."

Julia Cordua, the CEO of Thorn, said that Apple's technology balances "the need for privacy with digital safety for children." Thorn, a nonprofit founded by Demi Moore and Ashton Kutcher, uses technology to help protect children from sexual abuse by identifying victims and working with tech platforms.