WhatsApp connection is capable of decrypting your messages, given enough effort."
WhatsApp did not respond to an AFP query on the security claims. But some rival services say the Facebook-WhatsApp tie-up is likely to hurt confidence in the messaging app.
Nico Sell, co-founder of the security-focused app Wickr said it has seen "thousands more people than normal" downloading its app since Facebook's announcement.
"I think people will swap quickly out of WhatsApp now that it's part of Facebook," she said.
Sell said Facebook's core business is monetizing data, while Wickr aims at protecting user anonymity and privacy, by using top-grade encryption and paying bounties to hackers who discover any security flaws.