Industrial action by nurses and other health staff, which has led to the cancellation of thousands of operations and procedures, is set to escalate this week.
As the dispute over pay parity with the rest of the UK enters its second week, the Health and Social Care Board has warned patients there could be further cancellations of appointments and services from today.
Further action is planned by Unison, with the Royal College of Nursing also planning industrial action short of a strike.
The Belfast Health Trust said all its services would operate as normal today but has cancelled 900 routine outpatient appointments in the Royal Victoria Hospital, Belfast City Hospital and the School of Dentistry.
Health trusts have posted details of the latest updates online.
Patients with a cancelled appointment in any trust area have been told they will be contacted directly, otherwise they are advised to still attend as normal.
Letters of apology have been sent to patients along with new appointment dates.
All hospital surgical procedures, inpatient and day cases planned for today will proceed as normal in all Belfast Trust hospitals.
Outpatient appointments will also go ahead at Musgrave, Mater and the Royal Children's hospitals, Royal Jubilee Maternity Service and Northern Ireland Cancer Centre including Bridgewater Suite and Belfast City Hospital.
Nurses and nursing assistants at Altnagelvin Hospital will join their colleagues across Northern Ireland on the picket line today.
The Western Trust has warned of "significant" disruption across the majority of its hospital sites.
This includes a small number of Children's and Adolescents Mental Health Services (CAMHS) appointments being postponed, which the Trust said would be "prioritised and rearranged" as soon as possible.
Several daycare centres will also be closed today in Omagh, Dromore, Strathroy, Castlederg, Fresh Focus, Lackaboy and Tir Navar.
All outpatients and red flag day case appointments at Altnagelvin, Omagh and the South West Acute hospitals will proceed as planned while emergency services will not be affected.
In the South Eastern Trust there are no planned cancellations today but an update said "there may be some disruption as action short of strike action continues".
The health unions rejected a partial pay increase offer following talks with Secretary of State Julian Smith on Thursday after which Department of Health permanent secretary Richard Pengelly said he did not have the authority to restore full parity.
Unison regional secretary Patricia McKeown said the latest proposal by the Department of Health was rejected because it does not deliver pay parity.
She added: "The department is insisting that it cannot deliver pay parity in the absence of ministerial authority, a position that is not credible in the face of this growing crisis. We have written to the Secretary of State calling on him to use his influence to secure the full resources needed to secure pay parity."
Unison has also written to the leaders of the five main parties urging them to tell Julian Smith that the health service should not be used as leverage in advance of the political talks scheduled for next Monday, December 16.
Ms McKeown added: "Whilst we don't accept the department's position that they need ministerial authority to grant workers pay parity, we have also asked the party leaders to write directly to the department telling them to implement pay parity and resolve this dispute.
"If the department thinks it needs political cover then we don't see why this won't give that to them."
Meanwhile, the DUP minister accused of breaking pay parity has admitted he is "guilty as charged".
Former Health Minister Jim Wells said his decision was unanimously endorsed by the full Executive and backed since by subsequent ministers. He stressed his only alternative would have been to close wards, including cancer wards, and stop the delivery of expensive drugs.
"On all of this, on this occasion, I am guilty as charged. All the allegations that I did this are entirely true. It's as simple as that," he told Sunday Life.
"I make no bones about it. We had to balance the books."
The decision was endorsed by the Executive, and then by the next Health Minister Simon Hamilton followed by Michelle O'Neill and the Executive itself again.
"It is significant that Sinn Fein took exactly the same action when faced with the situation," Mr Wells added. "I don't think that we paid nurses particularly badly. We tried to be fair to staff."