Facebook-owned WhatsApp has brought legal action against Israel's NSO Group, which claims the company stood behind cyber attacks as infected devices with malicious software.
WhatsApp accuses the company of sending malicious software to about 1
Users affected included journalists, human rights activists, political dissidents and diplomats.
NSO Group, which makes monitoring software, disputed the allegations.
In a court filing, WhatsApp said that NSO Group "developed malicious software to access messages and other communications after being decrypted on target devices."
It said that NSO Group created various WhatsApp accounts and caused the malicious code was transmitted over the WhatsApp servers in April and May.
"We believe this attack targeted at least 100 members of civil society, which is an unmistakable pattern of abuse," WhatsApp said in a statement.
The affected users had figures from several countries, including Bahrain, United Arab Emirates According to the lawsuit, Emirates and Mexico.
WhatsApp said it is seeking a permanent injunction prohibiting NSO from using the service.
The company, which was acquired by Facebook in 2014, said it was the first time an encrypted message provider had taken legal action of this kind.
WhatsApp markets itself as a "secure" communications app because messages are ultimately encrypted. This means that they should only be displayed in legible form on the sender's or recipient's device.
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NSO Group said it would fight the allegations.  "In the strongest possible terms, we dispute today's claims and will fight them," the company said in a statement to the BBC.
"The NSO's sole purpose is to provide technology to licensed authorities intelligence agencies and law enforcement agencies to help them fight terrorism and serious crime."