The communications app used by U.S. President Donald Trump's former national security adviser, Mike Waltz, says it is temporarily suspending services following a reported hack that exposed some of its messages.

In an email, Portland, Oregon-based Smarsh, which runs the TeleMessage app, said it was "investigating a potential security incident" and was suspending all its services "out of an abundance of caution."

A Reuters photograph showed Waltz using TeleMessage, an unofficial version of the popular encrypted messaging app Signal, on his phone during a cabinet meeting on Wednesday.

Waltz was ousted the following day, a move that capped weeks of controversy over his creation of a Signal group to share real-time updates on U.S. military action in Yemen.

That chat drew particular attention because Waltz, or someone using his account, accidentally added a prominent journalist to the group.

Concerns over the security of Waltz's communications were further heightened, when it was reported on Sunday that a hacker had broken into TeleMessage's backend infrastructure and intercepted some of its users' messages.

Tech news site 404 Media said the hacker provided them with stolen material, some of which the news site was able to independently verify.

Smarsh did not immediately respond to a request for more detail about the breach.