Chinese hackers remotely accessed several U.S. Treasury Department workstations and unclassified documents after compromising a third-party software service provider, the agency said Monday.

The department did not provide details on how many workstations had been accessed or what sort of documents the hackers may have obtained, but it said in a letter to lawmakers revealing the breach that "at this time there is no evidence indicating the threat actor has continued access to Treasury information."

"Treasury takes very seriously all threats against our systems, and the data it holds," the department said.

"Over the last four years, Treasury has significantly bolstered its cyber defence, and we will continue to work with both private and public sector partners to protect our financial system from threat actors."

The letter described the hack as a "major incident."

The department said it learned of the problem on Dec. 8 when a third-party software service provider, BeyondTrust, flagged that hackers had stolen a key used by the vendor that helped it override the system and gain remote access to several employee workstations.

The compromised service has since been taken offline, and there's no evidence that the hackers still have access to department information, Aditi Hardikar, an assistant Treasury secretary, said in the letter Monday to leaders of the Senate Banking Committee.

The department said it was working with the FBI and the Cybersecurity and Infrastructure Security Agency, and that the hack had been attributed to Chinese culprits.

It did not elaborate.