Two lawyers at the court, and a source close to it who asked not to be identified, said it had disconnected most of its systems that can access the internet, that employees cannot access e-mail and that employees who are not working on site cannot access documents.
The ICC, based in the Dutch city of The Hague, first disclosed a "cybersecurity incident" on Tuesday, saying it was trying to ensure the tribunal's "core work" continued. A court spokesperson declined further comment on Thursday.
Hearings in the trial of two men accused of leading militias that attacked Muslim civilians in the Central African Republic resumed on Thursday.
However the court's livestream was unavailable and there was no sound in the galleries where the public and journalists can view proceedings, court staff told media.
"As the defence team we do have limited access to the court systems," lawyer Geert-Jan Knoops, who represents one of the suspects, Patrice-Edouard Ngaissona, told Reuters. Knoops was in court on Thursday.
Ngaissona has pleaded not guilty.
The ICC is the permanent tribunal established in 2002 to try war crimes and crimes against humanity.
(Reporting by Toby Sterling; editing by Mark Heinrich and Barbara Lewis)
By Stephanie van den Berg
