Hospitals have seen "a real rise in pressure" in Tier 4 areas including London and the South in the past days.
NHS Providers deputy chief executive Saffron Cordery, the organisation representing NHS trusts across the country, said the increase in demand for hospital services is due to the rise in coronavirus rates - but also patients arriving with other conditions.
She told BBC News: "We're seeing a real rise in the pressure for hospital services, but also other types of NHS services as well.
"Ambulance trusts in particular are coming under extreme pressure, as are community and mental health services.
"Part of the issue is many more people coming through the door with Covid, but also people coming through the door with other conditions as well."
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She added the worst affected areas are London and the south of England.
"We're seeing real pressure particularly in London and the South, and that comes from this added demand, but also staff absence," she said.
"It's localised at the moment, but we could see that growing, and I think it's really important to remember that what's going on in the community is also going on in healthcare settings."
Ms Cordery said the spread of coronavirus among communities "impacts hugely" on staffing levels across the NHS.
She also urged members of the public to stick to coronavirus restrictions on New Year's Eve and beyond.
Ms Cordery added: "The public should be under no illusions that this is one of the most challenging times for the NHS.
"It is vital that everyone heeds the new rules and restrictions in the run-up to the New Year and beyond.
"After all, it takes a lot longer to curb the spread of the virus than it does to contract it.
"But despite the difficulties the service is facing, there is hope.
"The vaccine is here and is being rolled out, hopefully to all vulnerable groups by late spring."
