There are "many thousands" of Covid-19 cases across Northern Ireland, Michael McBride, the chief medical officer, has said.
At Stormont's daily briefing, Dr McBride warned that the spread of coronavirus is a lot wider than statistics indicate.
Dr McBride said there will be more deaths to come.
Asked if the current lack of testing may be creating a false sense of security among the public, he said: “A test at a point in time is just that.
"It may indeed reinforce behaviours that we don’t wish to see.
“As we open up the Covid-19 centres, that does provide other opportunities for us to track the spread of the virus within the community, but we must prioritise those who are sick and critical care staff.”
Dr McBride said the health service would have the capacity to carry out 1,000 tests a day from next week.
Dr McBride said the first priority is that personal protective equipment (PPE) gets to "those staff in the front line" and re-emphasised the importance of adhering to hand hygiene and social distancing practices.
"Don't look back in two weeks' time and think we should have done more," he added.
Meanwhile, the NI Health and Social Care Board has appealed to retired GPs who want to help during the Covid-19 pandemic to contact them.
It is part of a wider appeal announced last Friday by the Department of Health to recruit former health professionals.
There will be a range of roles available to GPs, including telephone triage.
The board wants retired GPs to get send their contact details to firstname.lastname@example.org - they will then be contacted with registration information and help.
Retired doctor, Brian Patterson told BBC Radio Ulster’s Evening Extra programme that "the times are such, that there are things we can possibly help with".
"We've got to also recognise that we're not not in the first flush of youth and slightly more vulnerable than most," Dr Patterson said.
"That said, there's a variety of roles we can do. Some are clinical roles and some are non-clinical roles.
"Taking and making phone calls, the triaging; we can do a lot of work with prescriptions, we can do a lot of work with laboratory results to free up the GPs of today."
Health Minister Robin Swann said the executive is pushing ahead with plans for a "volunteer army", not just for the health service, but for the wider battle against Covid-19.
He said more than 30 local firms have come forward to help make ventilators, while more than 40 have volunteered to make personal protective equipment.
Mr Swann also said a new mobile app will be launched to advise the public on whether they have coronavirus symptoms.
Dr Tom Black from the British Medical Association said it was hoped 13 such centres would be operating within a week.
The centre at Altnagelvin Hospital will be the "middle ground" for the moderate to severe cases, Dr Black said.
Another GP, Dr Nichola Herron, stressed that the building is not a coronavirus testing centre and said there were "absolutely no testing facilities" there.
People have been encouraged to phone their GP as usual. They will then advise whether you should attend an assessment centre.