The US Treasury Department says Chinese government-backed hackers have likely broken into some of its systems.

US media say the department informed Congress of the cyberattack in a letter on Monday.

The department says no confidential documents were compromised by the unauthorized access. Calling it a major incident, the department says it's working to determine the impact together with the Federal Bureau of Investigation and other agencies.

In October, a Chinese hacking group reportedly targeted the mobile phone of now President-elect Donald Trump during the election campaign.

Mike Waltz, who was picked by Trump for national security advisor, said in a CBS interview earlier this month that it is necessary to impose "higher costs and consequences to private actors and nation state actors that continue to steal our data, that continue to spy on us."

Waltz is known as a hardliner against China. The rift between the US and China could deepen over cyberattacks.