According to the BBC the hackers were able to remotely install surveillance software on phones and other devices using a major vulnerability in messaging app WhatsApp.
The Financial Times says Israeli security firm NSO GROUP developed the attack.
WhatsApp, which is owned by Facebook, confirms that the attack targeted a select number of users, and was orchestrated by an advanced cyber actor.
The company says a fix was rolled out last Friday.
Yesterday WhatsApp urged all of its 1.5 billion users to update their apps as an added precaution.
The attack was first discovered earlier this month.