The Public Health Agency has identified a cluster of Covid-19 cases in the Limavady area.
The agency said it is linking them to a social gathering in a private or residential setting.
Testing and contact tracing is being carried out to identify anyone who may be potentially affected and to help prevent any further spread.
Health Minister Robin Swann said he had “long anticipated and warned that clusters will happen”.
The Department of Health’s latest dashboard shows nine of the 16 new cases confirmed on Thursday are in the Causeway Coast and Glens Council area, into which Limavady falls.
All new confirmed cases are monitored by the Public Health Agency to identify potential clusters, outbreaks or linked cases, with infection control advice offered where appropriate.
‘Long way to go’
In a statement, Mr Swann said people need to “guard against complacency”.
“The virus is still present in our community and must never be underestimated,” he added.
Chief Medical Officer Dr Michael McBride said “important strides” have been made in tackling the virus, however, “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,” Dr McBride continued.
‘We shouldn’t feel alarmed’
Analysis: Marie Louise Connolly, BBC News NI Health Correspondent
Clusters are to be expected and at this stage we shouldn’t feel alarmed.
We are all well used to the twists and turns of the pandemic and this is just its latest move.
While I reported on a recent cluster in Ballynahinch, this is the first time that the PHA has released detailed information about a specific cluster and has linked it to a social gathering in a house.
It is one of the side effects of coming out of lockdown.
As the goal posts shifted, most of us moved further and mingled with more people.
As the number of positive cases rise, that will undoubtedly impact the R number – which indicates the rate of transmission.
At the moment in NI that number is below 1 and that’s where we’d like it to stay.
However in the Republic of Ireland there are over 1,200 clusters which has pushed the R number to between 1.2 and 1.8.
That means any further opening of the country has been deferred to 10 August.
This doesn’t mean we are going backwards, instead we are standing still. In a pandemic, the best direction in which to be moving is always forwards.
Anyone concerned that they have symptoms of coronavirus, which include a raised temperature, a new continuous cough, or a change in their sense of taste or smell, can now be tested.
Tests can be booked at a number of sites around Northern Ireland by visiting the Public Health Agency’s website.
Meanwhile, in the Republic of Ireland, one further coronavirus-related death was reported on Thursday, with 21 new cases. The death toll there is now 1,749.