A council has called in police and cyber security officials after its emails were hacked.
Emails were brought down for several days at Newry, Mourne and Down District Council following the cyber attack.
It came after some accounts were compromised and began sending phishing emails encouraging recipients to follow a link to a file-sharing site.
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The council has warned the cyber attack "may have spread to others" due to emails sent during the online security breach.
All emails were blocked going in and out of the local authority for a period while the incident was investigated.
Email services were understood to have been unavailable between last Friday and Monday following the computer system hack.
The council took two days to respond to press queries about the incident. It is unclear if the delay was a result of the cyber attack.
In a statement, a council spokeswoman said: "Newry, Mourne and Down District Council has been impacted by a cyber incident.
"This may have spread to others by emails sent during a specified timeframe on June 1.
"All email recipients of the council have been notified as has the National Cyber Security Centre, the Information Commissioner's Office and the PSNI.
"Council services were able to continue as the council has robust business continuity plans in place in the event of an incident.
"The council has investigated the incident and continues to monitor and control IT security robustly."
Councils across Northern Ireland have been looking at ways to step up their online security following increased concerns over cyber threats.
Newry, Mourne and Down council earlier this year said they were dealing with "daily" attempted attacks on their computer systems.
In May, it emerged Derry City and Strabane District Council had restricted access to its website to UK and Ireland users only in a bid to fend off cyber attacks.
Last year, Queen's University Belfast had to suspend access to some of its systems as a precaution following a cyber threat.
Fears over the threat of cyber attacks were heightened last year after the Irish government's Health Service Executive was targeted by hackers.
Patient data was stolen and appointments were cancelled as IT systems were forced offline in the ransomware attack - the largest data breach in the Irish Republic's history.
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