The staffer, who works for Senator Eric Schmitt, shared the details of the briefing on condition that he not be identified by name.

Allegations that China hacked the State Department - along with two dozen other, mostly still unidentified organizations - have strained an already tense U.S.-China relationship; Beijing has denied being behind the spying.

The hack has also refocused attention on Microsoft's outsize role in providing IT services to the American government.

"We need to harden our defenses against these types of cyberattacks and intrusions in the future," Schmitt said in a statement shared by the staffer in an email to Reuters following the briefing.

"We need to take a hard look at the federal government's reliance on a single vendor as a potential weak point," he said.

The U.S. State Department did not immediately return a message seeking comment. Schmitt's office did not immediately respond to a request for an interview with him.

(Reporting by Raphael Satter; Editing by Leslie Adler)

By Raphael Satter