The agents seized the phones and an iPad earlier this week pursuant to a court authorized search warrant, a person familiar with the matter told CNN.

The search was first reported by the New York Times, which said the probe centred on whether the campaign had conspired with the Turkish government to receive illegal foreign donations, reported BBC.

The Mayor has denied all wrongdoings.

Boyd Johnson, a lawyer for the mayor's campaign organisation, said the FBI approached Adams.

"After learning of the federal investigation, it was discovered that an individual had recently acted improperly. In the spirit of transparency and cooperation, this behaviour was immediately and proactively reported to investigators. The Mayor has been and remains committed to cooperating in this matter,"  Adams' campaign attorney Boyd Johnson told CBS News.

"On Monday night, the FBI approached the mayor after an event. The Mayor immediately complied with the FBI's request and provided them with electronic devices. The mayor has not been accused of any wrongdoing and continues to cooperate with the investigation," Johnson said.