Senior officials from some 20 countries that are allied with the United States were reportedly the targets of a hacking campaign by the Israeli firm NSO Group that used the popular WhatsApp messaging service.

Citing people familiar with an internal investigation by the Facebook-owned WhatsApp, Reuters reported Thursday that a “significant” number of the victims were prominent government and military officials.

The report came days after WhatsApp filed a lawsuit in the US against NSO Group, accusing it of using the hugely popular instant messaging platform to conduct cyberespionage on nearly 1,400 journalists, diplomats, dissidents and human right activists worldwide.

The Israeli firm has denied journalists and activists were targeted and said that it only licenses its software to governments for “fighting crime and terror.”

The lawsuit said Pegasus was designed to be remotely installed to hijack devices using the Android, iOS, and BlackBerry operating systems.

The complaint said the attackers “reverse-engineered the WhatsApp app and developed a program to enable them to emulate legitimate WhatsApp network traffic in order to transmit malicious code” to take over the devices.

Its best-known product is Pegasus, a highly invasive tool that can reportedly switch on a target’s phone camera and microphone, and access data on it.