WhatsApp is working to secure a security fix after hackers could have injected surveillance software into a number of phones by making in-app phone calls.
The popular messaging app discovered a vulnerability that allows hackers to install code on both iPhones and Androids by ringing the targeted device.
Even if the user doesn't answer the call, the code can still be transmitted and the recording log of this call can often disappear, the Financial Times reported.
Attempts were reported to gain access to the phones of human rights campaigners, including a UK-based lawyer.
The social app , owned by Facebook, told the FT that the attack resembles the software developed for intelligence agencies whereby the spyware takes over the functions of the phone's operating system.
"We have briefed a number of human rights organisations to share the information we can, and to work with them to notify civil society," said a spokesperson from WhatsApp.
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According to the Financial Times, the spyware was developed by an Israeli cybersecurity and intelligence company, NSO Group.
However the company told the paper: "Under no circumstances would NSO be involved in the operating or identifying of targets of its technology, which is solely operated by intelligence and law enforcement agencies.
"NSO would not, or could not, use its technology in its own right to target any person or organisation, including this individual (the UK lawyer)."
WhatsApp, which claims to have 1.5 billion users around the world, notified the US Department of Justice after discovering the vulnerability in early May, and released a software update on Monday.
The vulnerability and suspected attacks have been investigated by Citizen Lab, a research group at the University of Toronto.
"We believe an attacker tried (and was blocked by WhatsApp) to exploit it as recently as yesterday to target a human rights lawyer," the lab said.
Amnesty International said it was backing legal action against the Israeli Ministry of Defence demanding that it revokes NSO Group's export licence.
Danna Ingleton, deputy director of Amnesty Tech, said: "NSO Group sells its products to governments who are known for outrageous human rights abuses, giving them the tools to track activists and critics."
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