The deaths of 11 people - including five babies - are under investigation by the Northern Health Trust.
Health care failures linked to the deaths within the trust area were not properly acted on, the Assembly has been told.
In an urgent written statement to MLAs, Health Minister Edwin Poots said issues had been identified in radiology, obstetrics and gynaecology, and accident and emergency.
Mr Poots said 20 instances in the period 2008 to 2013 had been brought to his attention by trust officials earlier this week. Of the cases, 11 involved a death. Five of the deaths were of babies who were either still born or died shortly after birth.
It is understood at least one of the 11 deaths was linked to a delay in diagnosis in an emergency department.
Others involved a failure to correctly follow up on issues identified in chest X-rays - in some cases a radiographer's request for a scan to be carried out was not acted on and, as a consequence, serious conditions were not diagnosed in time.
Problems were identified in the Causeway Hospital in Coleraine but the trust has ordered a review of X-ray reports produced in all its hospitals and has set up a helpline for concerned patients.
Mr Poots has stressed that the exercise was precautionary and there were no concerns about the accuracy of the X-ray reporting.
In obstetrics and gynaecology, concerns were identified about how serious incidents were managed.
The minister said: "It is not clear that these were avoidable deaths but it is clear that the trust's response should have been better."
Health officials have previously raised concern about under-reporting of so-called Serious Adverse Incidents (SAIs) within the Northern Trust.
Of the 20 instances highlighted by the minister, eight were not originally reported as SAIs. In many of those cases health care failures only emerged after family members complained or took legal action against the trust.
The performance of the trust has been under intense scrutiny in recent years and in 2012 Mr Poots ordered a so-called 'turnaround team' from England to start working alongside management to raise standards.
Concerns over SAIs have not been confined to the Northern Trust and last month bosses in the Belfast Trust were heavily criticised after it emerged that waiting times and staff shortages were contributory factors in the deaths of five patients in the Royal Victoria Hospital's A&E last year.
Mr Poots said the Northern Trust had acted to address the failings identified in the 20 cases.
"I want to assure the Assembly that the trust has now taken prompt and appropriate action such as initiating fuller investigations and making sure all affected patients and families are given all appropriate information and support," he said.
"I have asked the trust to confirm as soon as possible that all such action has been completed to ensure that these individual cases have all been reported appropriately, properly investigated and that learning from those instances is effected within the trust and more widely within the HSC (Health and Social Care) as appropriate."
The trust has reviewed 35,000 X-ray reports produced at the Causeway Hospital between 2011 and 2012 and nine cases have been flagged up as needing further investigation.
Almost 50,000 X-ray reports produced in other trust settings are also being examined. More than 19,000 have already been looked at - with concerns identified in two cases - and 28,000 are still to be reviewed.
Any patient with concerns about radiology is asked to call 028 9442 4804.
Mr Poots said it was vital when things went wrong they were reported.
"While the identification of an incident as an SAI does not in itself have any impact on the outcome for the individual patient at the time the incident occurs it is, as I have already highlighted, crucial that it results in a prompt and timely investigation so that any learning can be shared to ensure processes and procedures or other corrective action can be applied," he said.
The minister insisted the vast majority of patients in Northern Ireland received "very high quality" care.
"The fact is that in such a highly complex and stressful environment, no matter how committed or dedicated staff are, things on occasions, can and will go wrong for many reasons," he said.
"While this only applies in a tiny proportion of cases, to deliver a high quality service, it is vital that learning is achieved from all such events and applied consistently so as to minimise and to prevent in as far as possible, the risk of re-occurrence.
"There can never be room for complacency. Safety will always be the component of quality that needs to be guarded and continually improved and consistent and timely reporting is fundamental to that. The price of quality is eternal vigilance."