Dr Andrew Goodall, the chief executive of NHS Wales, has said there are around 2,870 patients being treated for COVID-related issues as critical care services operate at 152% of normal capacity.
"At that level, the NHS will have to make some very difficult decisions about the balance of services it can provide," he warned during the Welsh government's press briefing on Wednesday, adding that it would take "weeks" for the lockdown impact to be seen.
It would also be "a while yet" before the number of hospital admissions that ultimately end up in critical care would begin to fall.
There are currently 150 people being treated in these units across the country, with patients averaging around 59 years old.
Twice as many men than women also occupy these beds.
According to Dr Goodall, around 38% of people who are taken into critical care with COVID-19 ultimately do not survive, meaning more admissions equals more deaths.
He said: "The last few weeks have been sobering and extremely difficult for staff throughout the NHS. High levels of positive cases in the community leads to more hospital admissions, more seriously ill patients, including in critical care, and ultimately more deaths".
Wales's chief medical officer Dr Frank Atherton also spoke at Wednesday's briefing, and pointed to "encouraging signs" of transmission rates beginning to stabilise - but still urged people to be careful.
Rates still remain "particularly high" in Flintshire and Wrexham in North Wales, he said, which have been attributed to the disease variant still spreading through South Wales.
Despite this, the incidence rate has fallen in Wales from 650 cases per 100,000 in mid-December to 410 cases. Test positivity rate has also fallen from 25% to 20%, although Dr Atherton said this was still "way too high".
The doctor then noted a "good" trend in mobility data, which revealed there was a similar number of people travelling around their areas as was seen in the two-week firebreak lockdown.