The number of Covid patients in Wales' hospitals is now more than double the previous peak seen last April.
Welsh NHS Chief Executive Andrew Goodall said 2,870 Covid patients were being treated.
The effect of lockdown restrictions will take some weeks to be felt, he told coronavirus briefing.
There are some signs that coronavirus levels are starting to stabilise, although an official warned rates remained high in parts of north Wales.
Dr Goodall told Wednesday's press conference that - at current levels - the NHS would have to make "some very difficult decisions" about the services it could provide.
It was already the most challenging winter of his career, he said, adding it would be "a while yet" before the number of admissions to hospital began to fall.
"The impact of the restrictions, including the lockdown measures, will take some weeks to be felt in the NHS," he said.
A further 66 people are reported to have died in Wales with coronavirus, according to Public Health Wales, along with 1,533 new cases.
What are the latest hospital figures?
The number of Covid patients in hospital on 12 January was 2,879, 107 more than the week before. Numbers have been running at record levels
It had risen by 17% in the Betsi Cadwaladr heath board and reflects the pressures at Wrexham Maelor Hospital, which suspended routine surgery to cope.
Of those in hospital on 12 January, 1,610 were confirmed Covid-19 patients, 195 were suspected of having Covid and 1,074 were recovering from the virus and are too ill to be discharged.
This number is by far the highest level we've seen since then - but because recovering patient figures were not reported until the end of May, so it is a little difficult to compare figures with April.
Chief Medical Officer Dr Frank Atherton said there were some encouraging signs over the past few days "that coronavirus is beginning to stabilise in Wales".
There are 410 cases for every 100,000 individuals at the moment, down from about 650 per 100,000 in mid-December, he said.
But Dr Atherton struck a cautious tone, saying there were high rates in areas of Wrexham and Flintshire.
There are now 14 hospitals at level three or four - the two highest levels used by the NHS to measure pressure on services. Three hospitals are at level four.
Dr Goodall warned new variants of the coronavirus could lead to further increases in transmission in Wales in the coming weeks.
Public Health Wales said at least 30% of recent positive tests in south east Wales are of the new variant, with at least 70% of cases having the variant in north Wales.
Dr Goodall said his greatest concern was "the impact on critical care" with 150 people now in intensive care units with coronavirus.
"This is the highest level we have seen during the second wave. This is almost our entire non-pandemic critical care capacity," he said.
The average age of patients in critical care is 59, with almost twice as many men as women.
Meanwhile, 186 patients are in field hospitals, with that number increasing, Dr Goodall said.
He said staff were "exhausted" and sickness levels were higher than normal for this time of year.
About 9% were off sick, compared to the usual 5-6%.
Close to half of that was due to Covid-19 - either because staff were ill or had to self-isolate. Dr Goodall said it had been a "constant treadmill" for staff.
A senior Welsh doctor said there could be another four-to-five weeks of "serious issues" in hospitals.
Dr David Bailey, of the British Medical Association, said: "It's good that the numbers have started to drop, but that's going to take three, four, five weeks to work its way through to the medical workload at the sharp end, in medical units, in emergency admissions and in critical care.
"People stay in critical care for a couple of weeks; you develop a serious illness with Covid maybe 10 days after you start, so all of these things move down the way in terms of when they come through in the health service."
Paul Davies, leader of the Conservatives in the Senedd, said: "We need to roll out the vaccine as soon as possible because that's the way out of tackling the virus.
"327,000 doses have been received (in Wales) but only 101,000 people have received the vaccine and we are falling behind other parts of the UK."
The Welsh Government has said there has been a significant acceleration in the roll out.
'Unrealistic' to expect NHS savings
Meanwhile, Wales' Health Minister Vaughan Gething told a Senedd committee NHS bodies would not achieve the efficiency savings of previous years this year or next due to Covid-19.
He said it would be "wholly unrealistic and unfair" to expect 1% savings "during the middle of the pandemic".