The Welsh Ambulance Service has warned that queues at many of Wales' major hospitals are having a direct impact on its ability to respond to 999 calls.
Accident and Emergency departments have faced 'unprecedented' pressures, with exceptional number of admissions over the last few days.
Abertawe Bro Morgannwg University Health Board, which runs services in Bridgend, Neath, Port Talbot and Swansea, said many of the people it has been treating have cardiac problems or have suffered a stroke. Many are over the age of 85.
It and other health boards have cancelled many planned routine operations, redeployed staff to work in A&E departments and found extra bed space for emergency admissions.
The Welsh Ambulance Service said it is "very concerned at the handover delays that have been experienced recently".
It is "concerned that pressure across all hospitals in Wales has a direct impact on the ambulance response that is able to be provided to patients who call 999 for our help."
Health expert Marcus Longley warned that changes to the NHS in Wales, with fewer major A&E departments, could make the pressure on ambulances transferring patients worse, as response distances and times are increased.
Professor Longley said, though, that current problems lie within the system more broadly, and "the health service needs to get ahead of the problem, rather than just forever scrambling to catch up with it."
The Welsh Government has urged patients to "think before attending the Emergency Department or dialling 999."
People with minor injuries or illnesses are being asked to consider other options, to reduce pressure on A&E departments:
Self care: Keep a medical cupboard well-stocked, and rest at home
Community support: A pharmacist can provide expert advice, and provide medicine without an appointment
Routine care: An appointment with your local GP when it's not an emergency
Urgent care: If you need urgent healthcare, phone your GP surgery or out of hours service - for urgent dental care contact the NHS Direct Wales Dental Helpline on 0845 46 47
Emergencies: Call 999 and ask for the ambulance service for serious emergencies, or go to an A&E department