Facebook's WhatsApp on Tuesday urged users to upgrade to the latest version of its popular messaging app following a report that users could be vulnerable to having malicious spyware installed on phones without their knowledge.

"WhatsApp encourages people to upgrade to the latest version of our app, as well as keep their mobile operating system up to date, to protect against potential targeted exploits designed to compromise information stored on mobile devices," a spokesperson said. 

The Financial Times reported that a vulnerability in WhatsApp allowed attackers to inject spyware on phones by ringing up targets using the app's phone call function. It said the spyware was developed by Israeli cyber surveillance company NSO Group.

Asked about the report, NSO said its technology is licensed to authorised government agencies "for the sole purpose of fighting crime and terror", and that it does not operate the system itself.

"We investigate any credible allegations of misuse and, if necessary, take action, including shutting down the system. 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," the company said.

"NSO would not or could not use its technology in its own right to target any person or organisation."