Industrial action by healthcare workers is intensifying as Northern Ireland's nurses take part in 24 hours of action.
Health workers are industrial action in protest at pay and staffing levels which they claim are "unsafe".
In an unprecedented joint statement, the six health trusts said the action was likely to result in "a significant risk to patient safety".
Last week, the Royal College of Surgeons warned NI's healthcare system was "at the point of collapse".
On Tuesday, members of the Royal College of Nursing (RCN) are refusing to do any work that is not directly related to patient care.
Full details and advice on current health care services can be found on the Health and Social Care Board website.
Northern Ireland's Chief Medical Officer Dr Michael McBride called on all sides involved to step back, pause and resume negotiations.
"We now need to compromise, all parties need to compromise and I would appeal to all parties to compromise."
"Let's get around the table, let's find a way through this, let's find a way through that recognises that, at present, the department neither has the authority nor the budget to meet the trade unions' demands - the reasonable demands they are making."
Meanwhile, it has emerged that a nurse has said cancer diagnosis appointments continued at Belfast's Royal Victoria Hospital (RVH) despite claims by the Belfast Trust that they had been cancelled.
Patricia Cowan, a specialist dermatologist nurse, said she took part in those diagnoses on Monday.
The nurse said that about two thirds of booked patients turned up for appointments on Monday morning and, in the afternoon, only one person in a clinic booked for 12 people did not show.
"They were new patients who had waited for goodness knows who long for an appointment which was cancelled by the trust," she said.
In a statement, the Belfast Trust said it was "always aware that some staff would be available despite the industrial action".
But the trust said that it could not guarantee the required level of staffing to provide the outpatient appointments
"The decision to cancel appointments remains the only responsible course of action in the circumstances. Otherwise, large number of patients would be attending in circumstances where we could not guarantee they would be provided."
Speaking earlier on Tuesday, Dr Tony Stevens, chief executive of the Northern Health Trust said the system was "coping, but just coping".
"We have got a system that copes, that is safe, but it has very little tolerance. Our concern is the strike action will just tip us that little bit beyond where we're comfortable," he said in an interview on BBC NI's Good Morning Ulster.
"There is an increased risk that somebody is going to get missed or that something is going to happen."
Dr Stevens said he had "every sympathy" with health care staff and their frustration, but he said the Department of Health had been clear about its lack of resources.
'Breakdown in trust'
The trust chief called for a "political solution" to what he described as "a long-standing and now acute financial problem".
Former health chief John Compton told BBC NI's Talkback that there had been "a pretty profound breakdown in trust between the work force and the system".
"For me, this is a political decision at the end of the day," he said.
"The only political entity that we have at this point in time is the secretary of state. Therefore, I think it really is incumbent on the political system to respond to what is a growing and really quite unpleasant set of circumstances - of course for the staff - but mostly for the citizens of this country."
Ready to work
Speaking about the industrial action, Kevin McAdam of Unite, said: "If it has inconvenienced our public, we ask you to understand that this is your fight.
"We are fighting this fight for you for better services to make sure there is enough staff to deliver the healthcare you rightly expect."
He said he firmly believed there was money to pay health workers in line with their colleagues England, Scotland and Wales.
The RCN said it has about 16,000 members in Northern Ireland, where some 17,000 nurses and 5,000 nursing support workers are employed in total.
In November, nurses voted to take part in strike action for the first time in the 103-year history of the RCN union.
On its first day of industrial action - which started at 00:01 GMT on Tuesday and lasts for 24 hours - RCN members are not striking, but are refusing to do any task that is not directly "patient specific".
This includes things like:
- not working bank or overtime shifts
- not working unpaid hours
- not completing paper work other than individual patient records
- Not answering phones on wards
- Not doing administrative tests
- Not collecting prescriptions or pharmacy in the community
Two more days of industrial action, short of strike action, are to be held on 10 and 11 December.
The first day of full strike action by RCN members is set to happen the week before Christmas, on 18 December.
On Monday night, the Unite trade union said its members had also voted to strike on 18 December.
It said it has 4,000 members who work in the health service and they would join members of the Royal College of Nursing, Unison and NIPSA in strike action.
The chief executives of the health and social care trusts and the head of the Northern Ireland Ambulance Service said they recognised the dedication of health staff, as well as the need for "a long-term funding settlement that addresses service, workforce and pay pressures in a sustainable manner".
"The level of escalation... is causing us real concern with regard to service continuity during what is already a very demanding period," they said.
"We have been struggling with a system where funding has been made available on a yearly basis, which makes it impossible to plan for the long term.
"Demand is increasing and will continue to do so, meaning that the current system is simply unsustainable."
Where are the cancellations?
As a result of the industrial action, the Belfast Health Trust cancelled more than 10,000 outpatient appointments and surgeries this week.
The South Eastern Health Trust has also cancelled some services but on a smaller scale.
In Belfast, the following hospitals cancelled all outpatient appointments, day case procedures and planned surgeries on Tuesday and Thursday:
- Royal Victoria Hospital (including the children's hospital, maternity hospital and school of dentistry)
- Belfast City Hospital
- Mater Hospital
- Musgrave Park Hospital
However, some outpatient appointments are going ahead at these places:
- Health and wellbeing centres
- Community facilities
- Chemotherapy or radiotherapy at either the Bridgewater suite or cancer centre at Belfast City Hospital
- Macular clinic at Fairview
- Paediatric oncology at the Children's Hospital
On Wednesday, all outpatient services will run as normal. Planned surgeries and day case procedures will still be postponed.
Emergency services and day centres are not affected by the industrial action.
In the South Eastern Health Trust area, all routine outpatient appointments at the Ulster Hospital have been cancelled on Wednesday, with the exception of maternity and children's services.
On Friday, outpatient departments at Ulster Hospital, Lagan Valley Hospital, Downe Hospital, Ards Hospital and Bangor Hospital will be closed in the afternoon.