An outbreak of Covid-19 has rocked Major League Baseball, forcing the postponement of multiple games less than a week after the sport launched its season following a four-month coronavirus delay.

Major League Baseball confirmed in a statement that games between the Miami Marlins and Baltimore Orioles in Florida, and the Philadelphia Phillies and New York Yankees in Pennsylvania, had been scrapped.

The decision followed reports that 10 more members of the Marlins - eight players and two coaches - had tested positive for the coronavirus following Sunday's game against the Phillies in Philadelphia.

Four Marlins players - pitcher Jose Urena, first baseman Garrett Cooper, outfielder Harold Ramirez and catcher Jorge Alfaro - had already tested positive for Covid-19.

Major League Baseball said in a statement the games had been postponed in order to allow for additional testing.

"The members of the Marlins' travelling party are self-quarantining in place while awaiting the outcome of those results," the statement said.

In a separate announcement later Monday, MLB commissioner Rob Manfred said that the Marlins game against Baltimore on Tuesday had also been called off.

The Yankees were due to use the same locker room at Philadelphia's Citizens Bank Park that the Marlins had used over the weekend.

Marlins chief executive Derek Jeter said postponing Monday's game in Florida was the "correct decision." 

"The health of our players and staff has been and will continue to be our primary focus as we navigate through these uncharted waters," Jeter said.

"Postponing tonight's home opener was the correct decision to ensure we take a collective pause and try to properly grasp the totality of this situation."

Jeter said the Marlins had remained in Philadelphia to undergo further testing.

Baseball chief Manfred meanwhile said the Marlins could return to play as soon as Wednesday depending on the outcome of testing.

"Our first concern is the health of the players, and their families and making sure that we do everything possible to minimise the spread of the virus amongst our employees," Manfred told MLB Network.

"The Miami Marlins will not play tonight and tomorrow. We're doing additional testing. If the test results are acceptable, the Marlins will resume play in Baltimore on Wednesday."