A hospital in Northern Ireland has appealed to off duty staff to report for duty to help deal with increasing pressures caused by Covid-19.
Others have said they are extremely busy and have had to open additional beds.
A record number of confirmed coronavirus patients, 703, are being treated in Northern Ireland hospitals. That is almost double the peak number in April in the first wave of the pandemic.
In a joint statement, Northern Ireland's six health trusts warned that by the third week of this month hospitals could be dealing with double the current number of Covid-19 patients.
One of them, the Western Health Trust, issued a statement appealing to all off duty staff who work at the South West Acute Hospital in Enniskillen in Co Fermanagh.
"Due to increasing pressures this evening on the NI Healthcare system, we are appealing to you to go directly to the hospital," it said.
Dr Anne Kilgallen, chief executive of Western Trust, said Northern Ireland hospitals are facing into an abyss.
She said: "We can say that this situation is more grave than it has ever been in the course of this pandemic.
"I would go so far as to say our hospitals are facing into an abyss.
"At the moment one in four of the people in our hospitals have Covid-19. It's about 700 people. At the peak of the first surge there were 400 people in hospital so already we're in a very grave situation."
Dr Anne Kilgallen said health chiefs were asking people to work with them.
"We want to protect our emergency departments for the sickest people," she said.
The Southern Trust also issued a statement in which it said Craigavon Area Hospital in County Armagh and Daisy Hill Hospital in Newry, County Down, "are extremely busy."
The statement added: "Our staff have responded superbly and we have been able to open additional beds to support the high numbers of Covid-19 positive patients needing hospital care.
"Thank you to everyone who has reached out to offer help and support. The situation is currently very difficult but stable and we expect this high level of demand to continue over the next few weeks."
The chief executives of Northern Ireland's six health trusts again warned that the situation is very serious.
They said modelling projections indicate that in the third week in January "we will be trying to contend with double the number of Covid-19 positive patients compared to the current position today, when several hospitals already have record numbers of patients".
They said several of the Trusts have already stood down all but the most urgent elective surgery, including some red-flag cancer surgery, to redeploy staff to meet the urgent and immediate needs of extremely ill patients, especially both Covid-19 and non-Covid-19 patients needing ICU care.
It said no-one should attend an Emergency Department at any time unless they need emergency care.
They added: "Never has the phrase 'all in it together' been so pertinent and just so important.
"The Covid-19 vaccines provide the long-term hope and the current lockdown offers the opportunity to shorten the duration of the current surge. The public can play their part too by staying at home, practising social distancing and good hand hygiene and wearing face coverings."
Today saw a further 17 deaths due to Covid-19 and 1,112 additional positive cases in Northern Ireland.
It brings the total number of people who died of the virus to 1,460 and the total confirmed cases to 88,700 since the pandemic began.
There were 53 people with the virus in ICUs across Northern Ireland.
Hospital occupancy was at 94% and there were 15 ICU beds available.
The region's seven-day incidence rate per 100,000 of the population stands at 556.
Newry, Mourne and Down had the highest rate at 801 per 100,000 population, followed by Mid Ulster at 751 and Armagh City, Banbridge and Craigavon at 722.
The health chiefs said several trusts had already had to stand down all but the most urgent elective surgery, including some red-flag cancer surgery, to redeploy staff to meet the urgent and immediate needs of extremely ill patients needing ICU care.
"These postponed operations will be rescheduled as soon as possible," they said.
"We have established a regional approach to ensure that any available theatre capacity across Northern Ireland is allocated for those patients most in need of surgery, both during surge and as we come out of this surge.
"This may mean that patients will need to travel further for their surgery.
"Cancer services are seeking to maintain chemotherapy, radiotherapy and other non-surgical treatments and alternative treatments will be provided in the absence of surgical options."
They added that healthcare staff would do everything they could do to deal with the unfolding situation, despite being "exhausted".
"It will definitely not be easy and the care that we are able to provide will at times fall short of the high standards we normally deliver but we will do our very best," they said.
"Desperately ill patients whether Covid or non-Covid will always be the ones being prioritised."
The public has been urged not to attend emergency departments at any time unless they need emergency care.
"It is likely that those who do attend will wait longer to be seen and for admission to hospital if that is what they require," the chiefs said.
"Patients arriving by ambulance will also wait at times, sometimes for many hours before space is available in an already over-stretched ED."
They warned that when there is pressure in one part of the health and social care system "inevitably" it impacted on other parts.
"We might once again need families to be willing to fill unavoidable gaps in domiciliary care," they said.
"Never has the phrase 'all in it together' been so pertinent and just so important.
"The Covid-19 vaccines provide the long-term hope and the current lockdown offers the opportunity to shorten the duration of the current surge," they added.
They urged the public to continue to play their part by staying at home, practising social distancing and wearing face coverings.
New lockdown restrictions preventing people from leaving home except for essential reasons came into effect on Friday.
The strict rules will remain in place until 6 February but will be reviewed later this month.
Additional reporting PA