The US State Department said it is investigating an imposter who used artificial intelligence to impersonate Secretary of State Marco Rubio and contact three foreign ministers.

The "unknown actor" is alleged to have used an artificially-generated voice of Rubio to contact officials via the Signal messaging app, according to a State Department cable obtained by BBC's US partner CBS News.

The cable stated that the person contacted at least five individuals, including the foreign ministers, a US governor and a member of Congress.

The State Department said it is aware of the incident and is taking steps to improve its cybersecurity defences.

The incident was first revealed in the State Department cable that was dated 3 July and sent to "all diplomatic and consular posts," CBS News reported.

The cable stated that a false Signal account was created in mid-June with the display name marco.rubio@state.gov. That account contacted at least five people.

"The actor left voicemails on Signal for at least two targeted individuals, and in one instance, sent a text message inviting the individual to communicate on Signal," the cable stated, as reported by CBS.

The cable did not identify the individuals that were contacted or what the AI-generated voice of Rubio said in those voicemails.

"There is no direct cyber threat to the department from this campaign, but information shared with a third party could be exposed if targeted individuals are compromised," the cable said.

In a statement, the State Department said it is investigating the matter. It added that it "continuously takes steps to improve the department's cybersecurity posture to prevent future incidents."

The incident was first reported by the Washington Post on Tuesday. The newspaper reported that US authorities do not know who was behind the impersonations, but they believe the person's goal was to manipulate powerful government officials to gain access to information.

The Associated Press reported, citing a US official who spoke anonymously, that the hoaxes were unsuccessful and "not very sophisticated".

Secretary of State Rubio has not commented on the incident.

AI technology has been used in the past to impersonate US politicians.

Last year, a fake robocall claiming to be from former President Joe Biden had urged voters to skip the New Hampshire primary election ahead of the 2024 US election.

Officials in New Hampshire said at the time that the calls "appear to be an unlawful attempt to disrupt" the election, and that they were investigating the matter.