Northern Ireland public transport operator Translink has had its internal computer systems hacked, it has been reported.
The company said it was working to resolve the issue but it did not affect train or bus services.
It said there was no evidence data had been compromised.
The Irish News first reported of a ransomware virus cyber attack. Usually ransomware attacks block access to computers and demand payment.
The paper reported the internal networks had been out since Wednesday with some staff unable to log on to their computers.
Translink said it was experiencing difficulties with its internal IT systems and working to resolve the issue "as quickly as possible".
"Investigations are ongoing and there is no evidence that any data has been compromised," a statement added.
"Bus and train services are not affected.
"Passenger and timetable information is available on the Translink website, journey planner and through our contact centre."
Translink confirmed to the Belfast Telegraph the matter has been reported to the police.
The PSNI has been approached for comment.