With coronavirus cases popping up among multiple US sports leagues, the NFL Players Association has advised members to stop working out together.

That advice from union medical director Thom Mayer came on Saturday after Major League Baseball again shuttered spring training facilities in Florida and Arizona in the wake of a spate of positive Covid-19 cases for players and team staff members.

With the NBA, NHL and Major League Soccer all hoping to resume seasons shut down by the coronavirus pandemic, and MLB trying to hammer out a framework for getting 2020 underway, the cases have sparked concern.

Mayer said in a statement to NFL players that "no players should be engaged in practice together in private workouts."

The NFL is still in the midst of a "virtual" off season, with no formal team training sessions set to start until 28 July.

But with the league still aiming to kick off on schedule on 10 September, some players have been getting together for informal workouts as lockdown restrictions have eased around the country.

"Please be advised that it is our consensus medical opinion that in light of the increase of Covid-19 cases in certain states that no players should be engaged in practicing together in private workouts," Mayer's statement read.

"Our goal is to have all players and your families as healthy as possible in the coming months."

Multiple players from the Houston Texans and Dallas Cowboys have tested positive for the virus.

The NFL Network reported that a San Francisco 49ers player had tested positive and that he had been working out with team-mates in Nashville, Tennessee.

In the most recent reported cases, the Tampa Bay Buccaneers confirmed they had players or staff test positive, but declined to confirm reports that the positive results involved at least two players.

Quarterback Tom Brady, who signed with the Bucs this year after a glittering two decades with the New England Patriots, worked out with some of his new colleagues on a high school field in Tampa in May.

Florida, which announced a one-day record of 4,049 additional confirmed cases on Saturday, has become a worry-spot for sports trying to get their seasons on track.

Major League Baseball is shutting down all spring training facilities in Florida and Arizona - another state where coronavirus cases are on the rise.

The move comes a day after baseball's Philadelphia Phillies, Toronto Blue Jays and San Francisco Giants shuttered training campuses over virus concerns.

The Phillies said five players and three staff members had tested positive at the team's training centre in Clearwater, Florida. 

The NHL's Tampa Bay Lightning closed their facility 23 miles from Clearwater, after multiple players and staff members tested positive.

The rise in Covid-19 cases in Florida could be a serious concern for the NBA and MLS, which are both planning to resume play in so-called 'bubble' environments that will isolate multiple teams in the state.

The NBA is planning a 30 July restart at the Walt Disney World sports complex near Orlando, Florida.

The league plans to have 22 teams - 13 from the Western Conference and nine from the East - to start play with a chance to reach the final.

MLS is also planning a return at Disney World with its 26-team MLS is Back tournament, to be held without fans starting next month.