Chinese hackers who breached Microsoft's email platform this year managed to steal tens of thousands of emails from U.S. State Department accounts, a Senate staffer told Reuters on Wednesday.

Chinese hackers who breached Microsoft’s (MSFT.O) email platform this year managed to steal tens of thousands of emails from U.S. State Department accounts, a Senate staffer told Reuters on Wednesday.

The staffer, who attended a briefing by State Department IT officials, said the officials told lawmakers that 60,000 emails were stolen from 10 State Department accounts. Nine of those victims were working on East Asia and the Pacific and one worked on Europe, according to the briefing details shared via email by the staffer, who declined to be named.

The staffer works for Senator Eric Schmitt.

U.S. officials and Microsoft said in July that Chinese state-linked hackers since May had accessed email accounts at around 25 organizations, including the U.S. Commerce and State Departments. The extent of the compromise remains unclear.

U.S. allegations that China was behind the breach have strained an already tense relationship between the countries, as Beijing has denied the charges.