The Northern Ireland Ambulance Service (NIAS) is expecting a third night of staff shortages on Sunday, its medical director has said.
Dr Nigel Ruddell said about 20% of crews were unavailable, but measures were in place to address the issue.
He said voluntary ambulance services were being drafted in and NIAS had been engaging with private services.
Two ambulance crews were drafted in from the Republic of Ireland to help tackle the issue over the weekend.
The crews were based at Altnagelvin Hospital in Londonderry on Saturday and Newry ambulance station on Friday.
The service had appealed for staff to come to work at the weekend because of difficulties filling rotas.
“We are doing everything we can to provide as high a level of cover as we can,” said Dr Ruddell.
“We are going to prioritise those calls which have the most critical need.
“Unfortunately, those people who have a less critical need are the ones who wait longer, and we apologise for that.”
He also thanked staff for “stepping up to the plate” and volunteering to take on extra shifts.
The medical director said NIAS received about 600 emergency calls to its control room every day.
“We have 46 extra paramedics due to qualify in November and around 48 extra technicians who qualified in May,” he said.
“We are training and recruiting people as quickly as we can.”