COVID-19 hospitalizations in the United States have increased by about 33% and deaths are up by about 40% from a week earlier, the head of the U.S. Centers for Disease Control and Prevention (CDC) said on Wednesday.

CDC Director Rochelle Walensky, speaking on a media call, said U.S. COVID-19 cases, driven by the fast spreading omicron variant, are expected to peak in the coming weeks.

"The magnitude of this increase is largely related to the omicron variant, which now represents about 90% of the COVID-19 cases in the country," she told reporters.

Hospitalizations have been on the rise since late December as omicron surpassed the delta variant as the dominant version of the coronavirus in the United States, although experts say omicron will likely prove less deadly than prior iterations.

The recent increase in COVID deaths is likely a lagging effect of the delta variant, which was surging before omicron took hold in the United States in December, Walensky said.

With delta and other prior variants, deaths have lagged infection rates by a few weeks.

"We may see deaths from omicron but I suspect that the deaths that we're seeing now are still from delta," Walensky said, adding that it will take time to understand how omicron impacts coronavirus death totals.