A Chinese state-sponsored hacker has broken into the US Treasury Department's systems, accessing employee workstations and some unclassified documents, officials said on Monday.

The breach occurred in early December and was made public in a letter penned by the Treasury Department to lawmakers notifying them of the incident.

In the letter the Treasury Department said the China-based actor was able to override security via a key used by a third-party service provider that offers remote technical support to its employees.

The US agency characterised the breach as a "major incident", and said it had been working with the FBI and other agencies to investigate the impact.