The spread of highly contagious coronavirus variants is threatening to fuel a "potential fourth surge of cases" in the US, a top health official has warned.

The head of the US Centers for Disease Control and Prevention (CDC) said she was concerned by recent Covid-19 data.

She said about 70,000 new cases a day were recorded last week, "a very high number".

There were nearly 2,000 deaths a day in the same period, she said.

"Please hear me clearly: At this level of cases, with variants spreading, we stand to completely lose the hard-earned ground we have gained," Dr Rochelle Walensky, director of the CDC, said on Monday. "These variants are a very real threat to our people and our progress."

There are many different versions, or variants, of Covid-19 circulating, but health experts are particularly concerned about a few.

These include variants first detected in UK, South Africa and Brazil that appear to be more contagious.

The CDC has predicted the highly contagious B.1.1.7 variant first found in the UK will become the dominant strain in the US this month.