About 25 beds have been temporarily closed at Altnagelvin Area Hospital in Londonderry due to a vomiting bug and staffing shortages.
All non-urgent routine procedures "will be cancelled in the coming days", the Western Health Trust said.
It added there were a "number of ongoing challenges" and apologised for any inconvenience caused to patients.
Increased infection prevention and control measures have been put in place.
Ward 5 (Elective Orthopaedics) is completely closed due to the increased instances of vomiting and diarrhoea. The outbreak is affecting 6 wards in total.
The combination of infection control measures, staffing levels and delays in patient discharges means other beds in the hospital are also off-limits.
'Refrain from sitting'
Dr Dermot Hughes, the trust's medical director, said it was well-known that the hospital had problems recruiting staff.
"We have gone over to the Philippines, to Europe and to recruit outside of Northern Ireland. This has been partly successful," he explained.
"It's not simply that we have a shortage of nurses, we simply do not have enough staff applying for posts."
"There's probably a mismatch from demand and supply," he told BBC Radio Foyle.
"I think the universities could supply more and also we have our own problems in that we are further from the main centre of population which is the main Belfast area."
Dr Hughes said the situation was being reviewed on a daily basis to "ensure the most urgent scheduled operations and treatments are done".
"People should avoid visiting the hospital if they are feeling unwell, particularly if they have diarrhoea and/or vomiting," he added.
"Those who are visiting are asked to thoroughly wash their hands before and after visiting. Visitors should visit only one patient whilst at the hospital, refrain from sitting on hospital beds and not move from ward to ward when visiting."
Dessie Lowry from the Royal College of Nursing (RCN) said he believes there is a "fundamental issue in terms of the shortages of nurses that are available to staff the wards".
"That is the problem within the hospital, but not only in Altnagelvin, there is a significant issue elsewhere in Northern Ireland," he said.
Mr Lowry acknowledged that infection control is the responsibility of everyone who enters a hospital but said he believes staff shortages are a major factor in hospital beds having to be taken out of use.
"Beds have been closed… because there have not been enough nurses to ensure that safe and effective care can be delivered."
"This is an issue because successive governments haven't invested in nurse education," he added.
He said that the RCN would like to see a local Northern Ireland health minister back in place to ensure that enough nurses are trained and recruited post-Brexit.
The SDLP's Mark H Durkan told the BBC "routine procedures" were often "long-awaited and much-needed operations" and their cancellation would cause "huge disappointment" for patients.
He said while some of the issues were beyond the control of the trust or Department of Health, others were recurrent.
"We have an insufficient number of qualified staff and those that we do have are overstretched," he said. "This problem is only going to become more pronounced in the future unless action is taken now.
"Delays in patient discharges are often due to a lack of suitable home care packages to support discharged patients. This is an area in dire need of urgent and major investment."
Altnagelvin Area Hospital is an acute hospital, which has 472 inpatient beds and 36 day case beds.