A&E units are not coping and the "system is creaking", according to NHS England medical director Sir Bruce Keogh.
Speaking at a medical conference at the King's Fund, Keogh said ambulance services in certain areas of the country are struggling to cope.
He added that the resources of A&E centres are being pushed to the limit, especially over the winter months.
But he remains confident in the universal health service and said that with additional funding and better organisation, the NHS will be able to cope more easily than in winters gone by.
The government has ploughed an extra £300m into the NHS's winter kitty, 75% more than in 2013, for a total of £700m.
Dr Clifford Mann, president of the College of Emergency Medicine, added that in spite of the demand the NHS is facing, it is will functioning "pretty well".
On the other hand, he added that the launch of the NHS 111 helpline could increase the workload of NHS employees.
However, London Ambulance Service's director of operations, Jason Killens, said the new helpline is beneficial.
He told the BBC: "We have introduced a range of measures to help manage demand, including not sending an ambulance to more callers with less serious injuries and illnesses and instead giving them additional telephone advice or referring them to NHS 111."