The number of people waiting more than 12 hours in hospital emergency departments last month has almost trebled in Northern Ireland.
Flu, the vomiting bug norovirus and staff sickness challenged the system, the health service said.
The total number of emergency attendances waiting longer than half a day increased "markedly" from 888 in December 2016 to 2,372 last month.
The Health and Social Care Board said: "Whilst the increasing year-on-year number of patients attending emergency departments is an indication of rising demand for emergency department care, activity alone does not reflect the challenges that increasing acuity, complexity and needs of an ageing population place on the wider health system.
"Seasonal flu and other respiratory conditions, as well as norovirus at this time of year, has also presented challenges to the system, as has staff sickness.
"Regrettably, this means more people waited over 12 hours to be seen, treated and either discharged or admitted to hospital in the quarter, compared with the same period in 2016, as staff continued to prioritise the most clinically urgent patients."
Emergency departments and hospital wards in Northern Ireland were very busy from October to December last year, similarly across the rest of the UK and Ireland, the Board added.
Demand for emergency department care has seen the number of attendances rise by more than 13% between 2012/13 and 2016/17, its statistics showed.
Almost 8,000 more patients attended during the last three months of last year compared with the previous year.
More than two thirds were admitted, transferred or discharged within four hours of arrival, slightly down from the similar period the previous year.
There has been an improvement in the four-hour performance in the 2017/18 year to date compared with the same period in 2016/17 - from April to December 2017, 76% of patients were admitted, transferred or discharged within four hours, compared with 75% in the same period last year.
The Board said it was enhancing access to urgent and emergency care services.
"We are introducing new ways to care for people closer to home (like hospital at home initiatives), but we need to continue to reform the system in 2018 at pace and scale."
Roy Beggs, Ulster Unionist Assembly member, said it was a full-scale crisis.
"Nearly each new set of figures are setting the new record for being the worst in the history of the NHS, yet nothing is being done to address it.
"We still don't have enough beds, we still don't have enough staff and we still don't have enough adequate community care packages."