Almost 400 patients waiting for hospital beds
Almost 400 people were waiting for a hospital bed in Northern Ireland on Saturday afternoon, BBC News NI understands.
It is thought the majority of extra beds which are used in an emergency were already occupied.
Flu and respiratory infections have been affecting many people with some requiring hospital treatment.
BBC News NI also understands that 41 ambulances were queued outside Northern Ireland's emergency departments at 15:00 GMT on Saturday.
One ambulance had been waiting outside a hospital for nine hours.
There was not enough staff or space to facilitate crews or patients, it is understood.
A BBC reporter said at least seven ambulances were waiting to offload patients at Causeway Hospital in Coleraine on Saturday afternoon.
The Northern Ireland Ambulance Service (NIAS) said it was "experiencing severe challenges" as a result of the pressures being felt across the system.
"All calls received by NIAS are subject to triage to ensure that those who are sickest, receive our quickest response," a statement said.
"This will unfortunately mean delays for patients waiting with less urgent conditions and NIAS would apologise to those patients who find themselves in this situation."
Ulster Hospital
At the scene: BBC News NI reporter Darran Marshall
There are approximately 80 patients waiting in the ED at the Ulster Hospital at six o'clock.
Most seats are full and some patients are standing.
Around the corner at the ambulance bay, there are 10 ambulances parked up - each holding patients waiting to be dispatched.
One ambulance crew member told me there have been 10 different ambulances parked there since 07:00 GMT.
He said when he arrived on duty at 11:30, one crew had been parked up since 02:00.
Situation across Northern Ireland
BBC News NI contacted each of Northern Ireland's health and social care trusts for comment about the hospital bed situation.
The Western Trust said that on Saturday afternoon, 70 people were awaiting beds across their trust area.
This number included 22 people at the South West Acute Hospital in Enniskillen and 48 people at Altnagelvin Hospital in Londonderry.
The Belfast Health Trust said its latest figures showed a total of 38 patients were waiting for admission to a hospital bed on Friday.
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The Northern Trust said on X that it was "extremely busy today with long waits for those whose conditions are not life-threatening".