The chief executive of NHS in Wales has apologised to patients whose routine operations have been postponed due to “exceptional levels of demand” on the service in recent days.
Dr Andrew Goodall said December was the highest month on record for life-threatening “red calls”.
He added cancellations came as a result of trying to balance those pressures.
On Wednesday, the Royal College of Emergency Medicine Wales said emergency departments felt “like a battlefield.”
Dr Goodall told BBC Radio Wales this winter has been a “very challenging period for the NHS in Wales, there are significant pressures that the NHS is responding to”.
“The Welsh Ambulance Service Trust has reported that it’s had its highest month of red calls, which are its life-threatening calls, in December,” he said.
He added that New Years Eve saw a rise of almost 50% in the number of red calls they had to respond to.
“This morning as I’m speaking to you I know that there are 400 patients in our A&E departments across Wales, and around 50% of them are over 85,” Dr Goodall said.
“The NHS is responding to a very significant degree of pressure, and we have had winter plans in place to support this.
“It’s also very clear that staff have been fantastic on the ground in responding, they are very committed to the NHS and their response to patient care.”
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“I would want to reassure that there are a range of activities in place ranging from community services to extending hours, focusing on our out of hours services.
“We know that it is always going to be busier. There have been around 400 extra beds in the system.
“It is also important that we are adaptable to these pressures and we make sure we can respond to any increases in activity as they occur across Wales in individual sites.”
Of the cancellations, Dr Goodall said: “Any cancellation is a decision of last resort which is in the interest of trying to balance some of the emergency pressures alongside some of the planned care.”
Vanessa Young, director of the Welsh NHS confederation which represents health boards in Wales, said: “It has been particularly bad around the new year period.
“The Welsh Ambulance Service took 1,000 extra calls on New Year’s Day.
“The growth funding for the NHS has been less than in previous years – so we’re starting the year in a more difficult position.
“The funding we receive from Welsh Government is very welcome and we’re doing what we can to make the most of it.”
‘Completely unacceptable’
She added: “There is a question in society we need to ask about the type of NHS we want as society changes – we can’t expect it to respond when we’re being asked to do more and more each year with resources that aren’t growing at the same pace.”
Meanwhile Plaid Cymru leader Leanne Wood claimed that cuts in the number of hospital beds in Wales had put patient safety at risk, and urged the Welsh Government to take action.
She said it was “completely unacceptable” that the target of having no more than 85 percent of beds occupied in general and acute hospital services had not been met since 2011.
“Experts say that when occupancy is pushed above this level it poses a risk to patients because it can lead to the spread of disease or viruses,” Ms Wood said.
“Labour needs to reverse these cuts for the sake of patient safety.”