Washington: United States (US) have been warned of potential fourth surge of Covid-19 cases following the spread of highly contagious coronavirus variants.

According to details, the head of the US Centers for Disease Control and Prevention (CDC), Dr Rochelle Walensky has said that she was concerned by recent Covid-19 data.

“The country reported around 70,000 new cases per day last week which is a very high number. Covid-19 variants pose real threat to vaccine progress,” she said adding,

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

Although there are several variants of coronavirus, but few are said to be highly contagious. These variants include those first detected in United Kingdom (UK), South Africa and Brazil.

CDC had earlier issued warning that the B.1.1.7 variant first discovered in UK will become the most dominant strain in the US this month.

According to World Health Organization (WHO), US has been the worst hit country by coronavirus with more than 28 million infections and 500,000 deaths.

The country reported sharp fall in daily infections and cases after the vaccination drive started, but new variants and continuous spread pose serious threat to already under strained health systems of country.