A war of words has erupted between health chiefs and striking workers.
Unison's general secretary in Northern Ireland has disputed Belfast Health Trust's reasons for its decision to cancel diagnostic tests for people suspected of having cancer.
Patricia McKeown said the union would have exempted cancer services from their industrial action had they been notified it was an issue.
Martin Dillon, chief executive of the Belfast trust said they were forced to cancel the tests. He said the tests, which are carried out to tell patients whether or not they have cancer, will be rescheduled "as soon as possible".
He said it was a worry he did not know when the re-arranged appointments would take place.
Thousands of appointments and planned surgeries, procedures and admissions at the Belfast Trust were cancelled on Monday, Tuesday and Thursday as Unison workers started their second week of strike action.
Unions are taking the industrial action over unsafe staffing levels and the lack of pay parity with NHS workers in Scotland, England and Wales.
Speaking to the BBC, Patricia McKeown said the Belfast trust had not indicated any intention to cancel cancer diagnostic services.
"If it had done so it would have been exempted," she said.
"We ask the Belfast trust to urgently engage with Unison's local representatives to explain why they have taken this action and to put it right."
Ms McKeown said three of Northern Ireland's five health trusts had announced they would cancel all elective surgery some weeks ago, before industrial action was announced by her union.
"Right across the whole system, Unison members are working to rule.
"They are coming in to do their contracted jobs, they are stopping giving the tens of thousands of free hours they give to the health service every week in order to cover for missing colleagues."
Martin Dillon said he did not know when the cancelled diagnostic appointments could take place but the trust would "strain every sinew to reschedule those patients as soon as possible".
"Some patients whose GPs have referred them to us for diagnostic tests to discover whether they have cancer or not will not now have those tests this week because of the need to stand those patient services down," said Mr Dillon.
"I really worry about that."
Unison said its campaign was set to intensify but it remained open to further discussions with the departments of health, finance and the civil service.
The Secretary of State for Northern Ireland Julian Smith said he is concerned about the industrial action.
V concerned about strikes in NI health service today. In touch with health dept but as a devolved matter we need Stormont back to resolve— Julian Smith MP (@JulianSmithUK) December 2, 2019
Mr Dillon urged all stakeholders to engage in dialogue as soon as possible.
He apologised to all those patients who will miss surgery or appointments during the strike action.
"Given the scale of the industrial action we're facing this week, we had no choice but to stand down those services," he said.
"Unison more or less gave us the statutory seven days notice. It is Unison in particular that are taking extensive action this week. It's a combination of two unions taking slightly different approaches.
"Just over 10,000 patients won't have their outpatient appointment as scheduled and just over 1,000 patients won't have their surgery as planned this week.
"We have a clinical group making sure that red flag cancer services go ahead as normal."
However, this doesn't include patients who are waiting for essential tests to tell them whether or not they have cancer.
"Some of those patients would have been due to have tests this week which would confirm that one way or the other and regrettably we've had to stand those patients down," he added.
The chief executive confirmed this could lead to a delay in diagnosis and therefore a delay in treatment.
Last week, permanent secretary at the Department of Health Richard Pengelly asked trade unions to pause industrial action taken by health and social care workers.
He asked unions to enter into an independent conciliation process so a plan can be formed for any incoming Stormont Executive asking them to not allow "a bad situation to become worse".
"We are ready and willing to enter into a conciliation process with unions to map out an implementation plan for incoming ministers," he said.
A spokesperson for Unison said they are willing to engage in discussions, but only on the basis the resources needed to secure better working conditions are provided.
"We have proposed that the head of the civil service and his colleagues make direct representations to the Secretary of State for Northern Ireland and the UK Treasury to secure these resources.
"When such resources are made available we are fully prepared to re-engage in further discussions. In their absence our dispute with the health and social care employers will continue."