A cluster of COVID-19 cases has been identified by the Public Health Agency in the Limavady area.
The agency said it is linking this outbreak to a social gathering in a private or residential setting.
Testing and contact tracing is being carried out to identify anyone who may be affected by the virus.
Health Minister Robin Swann said he had warned that clusters will happen.
“I want to commend colleagues in the Public Health Agency for the work they are doing in keeping people safe through the Test, Trace and Protect programme.
“I would once again urge everyone across Northern Ireland to guard against complacency. The virus is still present in our community and must never be underestimated.
“Please stick to social distancing at all times and keep washing your hands regularly with the same rigour as at the start of this pandemic,” he added.
Chief Medical Officer Dr Michael McBride said the news underlines the need to be on the guard against COVID-19.
“Social gatherings in private residences have been an issue on both sides of the border.
“We have made important strides forward against the virus, but we must always remember that we have a long way to go. We face a very uncertain next few months and the very real prospect of a very challenging autumn and winter.
“Please keep following the public health advice at all times. Stay safe and save lives. And make sure you avoid crowds, closed spaces and close contact when you are socialising,” he added.