Health Secretary Jeremy Hunt has apologised to patients in England affected by a decision to postpone tens of thousands of operations in January.
Non-urgent treatments had already been cancelled until mid-January, but NHS England said on Tuesday that would now be extended to the end of the month.
It came after hospitals reported they were struggling to cope with the surge in patients being seen since Christmas.
Mr Hunt said it was "absolutely not what I want".
But he said the move was needed given the pressure hospitals were under.
"This is the busiest week of the year for the NHS."
And he also said the whole country was grateful for the work NHS staff were putting in working "incredibly long hours through the night, beyond the call of duty in every possible way".
His thanks were echoed by Prime Minister Theresa May, who also denied the health service was in crisis.
"The NHS has been better prepared for this winter than ever before," she added.
Reports have emerged of patients facing long waits for treatment and being stuck on trolleys in corridors, while ambulances are left queuing outside A&E.
It has prompted at least 16 hospital trusts to declare major incidents - sometimes known as black alerts - which can lead them to divert ambulances elsewhere and call in extra staff.
And some ambulance services have started asking 999 callers with less serious problems to make their own way to hospital so they can prioritise the most life-threatening calls.
Meanwhile, in Scotland there has been a 20% jump in A&E attendances compared with the previous year, prompting an increase in patients waiting over four hours, and in Northern Ireland the Antrim Area Hospital has been forced to bring in St John ambulance volunteers to help with a surge in demand.
NHS England's Prof Keith Willett admitted the pressures were severe - the worst he had seen since the 1990s - but said plans were in place.
As well as the cancelling of non-urgent treatments, such as knee and hip replacements, hospitals have been given the green light to put patients on mixed sex wards and to bring GPs into A&E to help deal with patients.
"A crisis is when you haven't got in place mitigations and you haven't got a plan to deal with it," Prof Willett said.
"We've gone into this winter in a way we've never prepared before."
Doctor warns of 'huge tragedy'
But Prof Suzanne Mason, of the Royal College of Emergency Medicine, said the measures were "too little too late" as hospitals simply had no beds free and these treatments would have had to be cancelled anyway.
She added: "Patient safety is being compromised - there's no doubt about that. When patients are in crowded emergency departments and staff cannot actually move between patients and provide the basic level of care that's required, then safety is compromised.
"Patients who spend many hours on a trolley - and these are often elderly patients - they are the sickest patients in our department.
"They are much more likely to have a poorer outcome and even die as a result of their experience in the emergency department. And that is a huge tragedy for us in our specialty and that's why we are so desperate to see things improve."
Reports have emerged of serious problems in a number of places over the past 24 hours:
- Nottingham's Queen's Medical Centre has asked patients to avoid its A&E after being deluged on Tuesday evening
- Southend Hospital said it was dealing with an "internal critical incident" with all its beds full, which has led them to call in extra staff
- A consultant at University Hospitals of North Midlands NHS Trust apologised for "third world conditions" in his hospital department
- Milton Keynes University Hospital is telling people only to attend for emergency treatment
- Two ambulance trusts in the east and north-east of England are on the highest alert
- A concentration of major incidents declared at hospitals across the West Country and South East
Doctors and nurses have also been speaking about their own experiences.
Dr Adrian Harrop, an A&E doctor at Scarborough Hospital, said he felt he was "fighting a losing battle" as he was not able to do his job properly and care for his patients in the way he wanted.
Meanwhile, Mark Nevison, a senior nurse in the north-east, tweeted he had worked in A&E for 10 years and had "never been so ashamed of the sub-standard care" now being offered.
Why has this happened now?
The first week of the year is always difficult.
The lack of availability of community services, such as GPs, over the festive period means hospitals tend to see a surge in really sick patients at the turn of the year.
Respiratory illnesses also tend to spike after families have been mixing over Christmas bringing frailer older relatives in contact with young family members, increasing the risk of infections being passed on.
But it is also true to say that this is part of a pattern. Last January was the worst in a generation and that followed the previous worst the year before.
Labour shadow health secretary Jonathan Ashworth has blamed this on "Tory underfunding".
The health service is in the middle of its toughest cash settlement since it was created.
How bad is the situation?
It is very hard to tell. The performance stats - covering waits in A&E, the number of ambulances queuing outside A&E and the amount of operations that have been cancelled - will not be known for a few weeks.
In the lead-up to Christmas, all the indications were that the NHS was in as bad a position as it was the previous winter.
Twice as many patients as there should have been were waiting for more than four hours in A&E, while bed occupancy rates were well above safe levels.
But last winter the really bad spell only lasted a couple of weeks before the pressure eased.
What should patients do?
The public are being urged to play their part by using the health service responsibly.
NHS England said calling 111 was often a quicker and more convenient way of obtaining clinical assessment and advice in non-emergencies and allowed staff in A&E to focus on the sickest patients.
The Royal College of GPs has also set out three basic steps that all patients should consider before seeking an appointment with their GP for an acute illness including self-care, using online guidance from NHS Choices and consulting with a pharmacist.