A senior department official told The Associated Press (AP) on Sunday that a cyber-attack had hit the network forcing technicians to take the system offline and repair the damage.

The official, who spoke on the grounds of receiving anonymity, said on Sunday that “activity of concern” was detected on the system around the same time as the White House computer network was compromised back in late October.

At the time, there was no evidence that the State Department had been affected although a number of other agencies – such as the US Postal Service and the National Weather Service – reported attacks.

The State Department's classified systems have not been affected with the unclassified email system due to return online on Monday or Tuesday as soon as the security improvements have been completed.