Federal authorities in the United States have said fully vaccinated people can resume recreational travel in the country at "low risk" and signaled a relaxation of operational guidelines for cruise ships, handing a major boost to the nation's battered tourism industry.

Releasing its long-awaited travel guidance on Friday, the US Centres for Disease Control and Prevention (CDC) said vaccinated individuals do not need a Covid-19 test and do not need to quarantine when traveling domestically.

However, they should still wear a mask and avoid crowds. In a separate statement, the CDC set out initial recommendations to help get cruise ships sailing again, including a timeline for vaccinating crew and updating a color-coding system to classify whether a vessel should be clear to leave port.

The advice comes as the pace of inoculations in the US quickens - more than 100 million people have been given at least one shot - even as more contagious variants threaten to ignite a new wave of Covid-19 cases and as millions of Americans take to the skies anyhow.

Carriers including United Airlines are already flying at as much as 80 percent of capacity and 1.6 million people passed through Transportation Security Administration checkpoints last Thursday, well above the 124,000 who did so a year before.

For international travel, people do not need to be tested for Covid-19 unless it is required by their destination country, and do not need to quarantine once back in the US, but should be tested before boarding a return flight.

The head of the CDC said she still recommends against non-essential travel - even for vaccinated people - but that the agency has a duty to update its guidance as more science becomes available.

"We know that right now we have a surging number of cases. I would advocate against general travel overall," CDC director Rochelle Walensky said in a briefing. "Our guidance is silent on recommending or not recommending that fully vaccinated people travel; our guidance speaks to the safety of doing so. If you are vaccinated, it's lower risk."

She also urged people to get vaccinated when they can.

"We all want to return to the things that we love. Getting more people vaccinated as quickly as possible and taking preventative measures to stop the spread of Covid-19 is the path out of this pandemic and back to our everyday activities," Dr Walensky said.

Covid-19 cases have started rising again in the US, with 79,000 new cases recorded on Thursday.

Mr Roger Dow, president and CEO of the US Travel Association, described the CDC's new guidance as "a major step in the right direction" in an e-mailed statement. He said the acknowledgement that vaccinations eliminate the need for testing and quarantine removes a key barrier to domestic travel.

The CDC still discourages non-essential domestic travel by those who are not fully vaccinated. They should get tested one to three days before travel, and again three to five days afterwards.

They should stay home and quarantine for seven days after travelling, or 10 days if they do not get a test, the CDC said.