Spyware crafted by a sophisticated group of hackers-for-hire took advantage of a flaw in the popular WhatsApp communications program to remotely hijack phones, the company said late Monday.

The Financial Times identified the actor as Israel's NSO Group, with WhatsApp describing the hackers as "a private company that has been known to work with governments to deliver spyware." A spokesman for the Facebook subsidiary later said: "We're certainly not refuting any of the coverage you've seen."

The malware was able to penetrate phones through missed calls alone via the app's voice calling function, the spokesman said. An unknown number of people were infected with the malware, which the company discovered in early May, said the spokesman, who was not authorized to be quoted by name.

John Scott-Railton, a researcher with the internet watchdog Citizen Lab, called the hack "a very scary vulnerability."

"There's nothing a user could have done here, short of not having the app," he said.

The spokesman said the flaw was discovered while "our team was putting some additional security enhancements to our voice calls" and that engineers found that people targeted for infection "might get one or two calls from a number that is not familiar to them. In the process of calling, this code gets shipped."

WhatsApp, which has more than 1.5 billion users, immediately contacted Citizen Lab and human rights groups, quickly fixed the issue and pushed out a patch. He said WhatsApp also provided information to U.S. law enforcement officials to assist in their investigations.

"We are deeply concerned about the abuse of such capabilities," WhatsApp said in a statement.

NSO said in a statement that its technology is used by law enforcement and intelligence agencies to fight "crime and terror."

"We investigate any credible allegations of misuse and if necessary, we take action, including shutting down the system," the statement said. A spokesman for Stephen Peel, whose private equity firm Novalpina recently announced the purchase of part of NSO, did not return an email seeking comment.

The revelation adds to the questions over the reach of the Israeli company's powerful spyware, which takes advantage of digital flaws to hijack smartphones, control their cameras and effectively turn them into pocket-sized surveillance devices.

NSO's spyware has repeatedly been found deployed to hack journalists, lawyers, human rights defenders and dissidents. Most notably, the spyware was implicated in the killing of Saudi journalist Jamal Khashoggi, who was dismembered in the Saudi Consulate in Istanbul last year and whose body has never been found.

A poster featuring a picture of Saudi journalist Jamal Khashoggi is held outside the Saudi Consulate in Istanbul, Turkey, on Oct. 25, 2018. Osman Orsal / Reuters, file photo

Several alleged targets of the spyware, including a close friend of Khashoggi and several Mexican civil society figures, are currently suing NSO in an Israeli court over the hacking.

On Monday, Amnesty International — which said last year that one its staffers was also targeted with the spyware — said it would join in a legal bid to force Israel's Ministry of Defense to suspend NSO's export license.