The top US infectious disease expert has warned that although the Omicron coronavirus variant seems less likely to cause severe illness, a surge in cases could still pose a heavy burden on the country's healthcare system.

The United States is now reporting a weekly average of about 550,000 new coronavirus cases per day. Hospitals are admitting about 15,600 new patients daily and reporting about 1,200 deaths a day.

In a video conference on Wednesday, White House Chief Medical Advisor Anthony Fauci blamed the surge on the highly transmissible Omicron variant.

He said multiple sources indicate a decreased severity with Omicron.

But he said the sheer volume of the number of cases "could still stress our hospital system."

Fauci urged the public not to pull back from recommendations for vaccinations, booster shots and masks.

The state of Maryland declared a state of emergency on Tuesday as hospitalizations topped 3,000 and hit a record high.