The South Eastern Trust has confirmed they are operating 20% above capacity.
It is understood some patients waited in A&E for more than four hours on Monday night.
They had been brought by ambulances on 999 calls, but with no beds in the hospital, those in need of medical attention were left on stretchers in a corridor.
It is believed that at one stage, seven ambulance crews were waiting to pass their patients on to doctors. That meant crews could not go out to other emergency calls.
Alliance health spokesperson Kieran McCarthy said he was very concerned at the delay.
"Ambulances are not meant to be tied up at the front door of a hospital, they should be responding to emergency calls. Lives are going to be put at risk if this problem is not resolved," the Strangford MLA said.
"The Health and Social Care Board should urgently look into what happened on Monday night and ensure that the resources are put in place so that it is not repeated."
He said the events on Monday night highlight that more must be done to cut waiting times at Belfast's emergency departments.
However, Mr Poots told UTV: "Patients lives were not put at risk - and that is the first thing we have to make very clear.
"People's lives aren't being compromised. People who needed to see doctors in an emergency situation - that happened."
The DUP minister said that those who didn't require immediate treatment had to wait.
"It wasn't satisfactory, it's not ideal. I'm not defending it. It's an area we need to do better in."
The Ulster Hospital's A&E has seen an extra 10,000 patients since the closure of the emergency department at Belfast City Hospital in 2011.
A South Eastern Trust spokesperson said: "The emergency department at the Ulster Hospital was extremely busy [on Monday] with 66 ambulances attending, 20% above capacity, and 247 new patient attendances."
The number of patients at the emergency department has increased this year because of the changes within the greater Belfast system - including the closure of the City's A&E, the spokesperson added.
There is increased pressure on our staff who continue to provide excellent care to our patients under very challenging circumstances.
South Eastern Trust
"The large number of ambulances resulted in a longer wait for ambulance turnaround yesterday but this would be out with our normal performance.
"Patients were triaged as normal and on occasion had their treatment started whilst they waited for an appropriate space to become available in the emergency department. Medical and nursing staff in the emergency department worked tirelessly to address patient needs at that time. "
A spokesman for the Ambulance Service said 10 patients waited more than two hours to be treated at the Ulster Hospital on Monday. Two were in the emergency department for longer than three hours before they were treated.
In January, Department of Health figures show more than 6,500 people attended the Ulster Hospital's A&E, with 223 waiting longer than 12 hours for medical attention.
The A&E at the City Hospital had its doors temporarily closed in 2011 because of a shortage of doctors.
But earlier this month, the Health Minister revealed it could shut permanently. Politicians are once again voicing their concern.
Roy Beggs, Ulster Unionist Health spokesperson, said that it was clear the strain of the closure has become too much.
"Paramedics have the vital responsibility of transporting seriously ill people to hospital; they therefore cannot afford to spend exorbitant periods of time sitting in queues in their vehicles or in the corridors of our hospital buildings," the East Antrim MLA said.
"The strain which the closure of the A&E department at Belfast City Hospital has placed on neighbouring hospitals is now having a visible knock on affect."
According to Margaret Ritchie, SDLP MP for South Down, the decision to close the Downe Hospital's consultant led A&E between the hours of 10pm and 8am has added to problems.
"Back in October 2012 I alerted the Health Trust to the situation whereby my constituents had to wait overnight in the Ulster Hospital A&E to see a doctor, after being referred from the GP Out of Hours service at the Downe Hospital.
"The authorities were notified that this was a situation getting out of control, that the decision to close the Downe A&E between 10pm and 8am was placing pressure on the Ulster Hospital."
The MP said that she made it clear then that steps needed to be taken.