WhatsApp has rushed to roll out a security fix after concerns were raised hackers could inject surveillance software on to phones via the call function.
The app discovered a vulnerability that allowed attackers to install malicious code on iPhones and Android phones by ringing up a target device.
The code could be transmitted even if users did not answer their phones and a log of the call often disappeared, the Financial Times reported.
The company, which is owned by Facebook , said the attack bore a resemblance to spyware developed for intelligence agencies.
Users should update their mobile app as soon as possible to ensure they have the latest version of WhatsApp.
There are concerns that the software was used in attempts to access the phones of human rights campaigners, including a UK-based lawyer.
WhatsApp told the FT: "We believe a select number of users were targeted through this vulnerability by an advanced cyber actor.
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"This attack has all the hallmarks of a private company known to work with governments to deliver spyware that reportedly takes over the functions of mobile phone operating systems.
"We have briefed a number of human rights organisations to share the information we can, and to work with them to notify civil society."
The firm is said to have alerted officials at the US Department of Justice after discovering the vulnerability in early May.
WhatsApp claims to have 1.5 billion users around the world and it released a software update on Monday.
