Vaccinated people in parts of the US where the delta variant is spreading rapidly should resume wearing face masks, the country’s top health authority after previously scrapping the mask mandate.

President Joe Biden said the announcement showed that America needs to "do better" on vaccinations, adding that a mandate for the country’s more than two million federal workers was now "under consideration".

Rochelle Walensky, the Centers for Disease Control and Prevention (CDC) announced the mask decision on Tuesday citing new data which shows that while vaccines remain highly effective, rare breakthrough cases involving delta have an increased risk of onward transmission.

"In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings," she said.

According to the latest CDC data, much of the southern US is experiencing high or substantial transmission, while highly vaccinated parts of the Northeast are mostly experiencing moderate rates of community transmission.

Substantial is defined as being between 50 to 100 daily cases per 100,000 people over a seven day average, while high is defined as more than 100 daily cases per 100,000.

In a statement following the move, Mr Biden said he would lay out new steps on Thursday to overcome a lag in vaccinations, which have stalled badly for months despite the fact the country has the highest supply of any country.

CNN reported the president would move to make having the vaccine a requirement for all federal employees and contractors.