Apple has begun alerting people whose phones were hacked by NSO's spyware, which essentially turns handsets into pocket spying devices and sparked controversy this year after reportedly being used on activists, journalists and politicians.

"On top of the independent investigation, NSO will cooperate with any relevant government authority and present the full information we will have," the firm said in a statement.

NSO said it has not confirmed its tools were used, but opted to "terminate relevant customers' access to the system" due to the seriousness of the allegations reported by Reuters and the Washington Post.

The Post reported that Apple alerted 11 US diplomats that their iPhones were hacked in recent months, citing people familiar with the notifications who said the attacks focused on officials working in Uganda or east Africa.

NSO Groups spyware has been engulfed in scandal since reports that Pegasus was used by foreign government clients to target the phones of human rights activists, embassy employees and others.

Apple sued the firm last month seeking to block NSO from using the Silicon Valley giant's services to target the over one billion iPhones in circulation.

Reuters, citing four people familiar with the matter, said nine American diplomats were targeted and added the intrusions represented the widest known hacks of US officials using NSO technology.

Apple declined to comment on the reports.

Just weeks before the Apple lawsuit, US authorities blacklisted NSO to restrict exports from American groups over allegations the Israeli firm "enabled foreign governments to conduct transnational repression."

Smartphones infected with Pegasus are essentially turned into surveillance devices, allowing the user to read the target's messages, look through their photos, track their location and even turn on their camera without them knowing.

Concern over Pegasus spyware further grew after Apple revealed in September it had patched a weakness that allowed NSO's spyware to infect devices without users even clicking on a malicious message or link.

The so-called "zero-click" attack is able to silently corrupt the targeted device, and was identified by researchers at Citizen Lab, a cybersecurity watchdog organization in Canada.

Apple said at the time it filed the lawsuit in a California federal court that it would notify the "small number" of users that it discovered may have been targeted by those types of attacks.