The Northern Health and Social Care Trust warned that anyone who attends either of the two hospitals will face a long wait to be seen unless their condition is urgent or life-threatening.
A spokesperson for the trust apologised for the situation.
“We apologise to everyone waiting. Please be patient and kind to staff who are working as hard as they can.”
The Western Health and Social Care Trust, meanwhile, has described the Emergency Department at Altnagelvin Hospital as being under “extraordinary” pressure.
“Our Altnagelvin Hospital has now reached 100% capacity. Our Emergency Department is now under extraordinary pressure, with many patients experiencing long waits for a bed,” a Trust spokesperson said.
“We are again asking the public to only attend ED in a medical or mental health emergency and to attend alone where possible.
“If your situation is life-threatening, a medical or mental health emergency, call 999 immediately or proceed straight to your nearest Emergency Department.
“If you have an urgent, non-emergency medical situation, then we ask that you utilise the Phone First service on 0300 020 6000 where medical personnel will direct you to the best route of treatment. This may include visiting a Minor Injuries Unit or out-of-hours GP.”