Accident and emergency services are already under strain from increased winter demand , the medical director of the NHS in England has admitted .
Speaking at a conference in London, Sir Bruce Keogh said: "The system is creaking, it is under pressure at the moment.
"A&Es are having to address increasing demand, the ambulance services are struggling in many parts of the country and we have a number of issues to deal with, which we are tackling."
He said forward planning meant the health service should cope better with a spike in demand for accident and emergency services this winter.
Measures included a flu plan announced in April and a system to help trusts which were struggling to plan for winter, launched in June.
A £400 million injection of funding from the Department of Health in June also helped preparation, he said.
Another £300 million boost in October helped with staffing and bed capacity.
Sir Bruce said: "We've started to address winter much sooner than in any of the other years.
"In winter we know that the demand on A&E changes, so while the activity might be slightly lower than during the summer, the proportion of people who need admission is higher."
He said that high expectations from the public and budget cutbacks were also making it more difficult for the NHS to succeed.
Sir Bruce defended proposals to overhaul the health system to create "supercentre" specialist A&E departments.
The Keogh review, published last November, advocated the concentrating of accident and emergency services into a few specialist hospitals.
He said: "There has been a slow and inadvertent deception that all A&Es are equal. They are not, and people in the know know that."
He also said that demographic changes has placed greater strain on the NHS and that younger people had "an expectation of an immediacy of service".
Dr Clifford Mann, president of the College of Emergency Medicine, speaking at the same conference, said: "I think the system is under pressure but it's working pretty well."
He said that increased pressure was partly due to ambulances being called by NHS 111 operators.