US President Joe Biden said on Friday "in all probability" new guidelines or restrictions would be imposed in the United States in response to a resurgence of Covid-19 cases.

Asked if Americans should expect new recommendations from health authorities or new restrictive measures, the president responded, "in all probability," before leaving the White House by helicopter for the weekend.

He did not specify what steps could be taken.

US federal authorities, local officials and businesses have boosted health protocols in recent days in the face of surging cases spurred by the spread of the highly contagious Delta variant.

Biden added, however, that the country had had "a good day" on Thursday in terms of vaccinations.

"Almost a million people got vaccinated," he said, as his administration works to revive a sluggish inoculation campaign.

"I am hopeful people are beginning to realize how essential it is."

US health authorities this week recommended that even vaccinated Americans again wear masks indoors in areas with high infection rates.

The federal government has also tightened health regulations for its millions of employees, who must now either be vaccinated or wear masks and be tested regularly, even in areas with low case numbers.