US Democratic presidential candidate Hillary Clinton has won the Missouri primary after her rival conceded the contest, a spokesman for Bernie Sanders says.

Results for the contest, which was held on Tuesday, were slow to roll in because of the razor-thin margin separating the number of votes received by Mrs Clinton and Mr Sanders.

Ultimately, Mrs Clinton's win was declared by just 1,531 votes.

"I prefer to save the taxpayers of Missouri some money," Mr Sanders said, according to spokesman Michael Briggs, declining the option to request a recount.

The winner is awarded just two additional delegates of the otherwise nearly evenly split spread.

The latest result means Mrs Clinton won in all five states where primary voting was held on Tuesday, including the key ones of Florida and Ohio.

Reuters/AFP