Extra staff have been called in to Belfast's Royal Victoria Hospital where the Accident and Emergency department has been described as "jam-packed".
The Belfast Health Trust has had to trigger several escalation measures tonight (Monday) as pressure increases.
About 30 people are waiting on hospital trolleys and there are over 100 people waiting in the emergency area, it is understood.
Some have reportedly been waiting since 10pm last night.
Extra beds have been opened and more staff have been called in as senior managers in the department monitor the situation.
The trust said there were no plans to call a major incident at this stage. In January a major incident was declared when the number of people waiting on trolleys reached 42.
SDLP MLA Fearghal McKinney said he was seriously concerned: “This is another symptom of the problem, but not the cause of problems within the health service. The (Health) Minister must listen to unions, staff and the public and must act on the root causes of the problem.
“Problems include extended GP waiting times, too many people presenting at A&E as a result - approximately 30% - and strategic decisions which failed to put money into community while removing vital hospital resources.”