Boxing legend Muhammad Ali is close to death in a US hospital, a source close to the family has told the Reuters news agency.

"It's extraordinarily grave. It's a matter of hours," said the source, who spoke on condition of anonymity.

Ali, 74, one of the world's most celebrated athletes, was hospitalised this week for a respiratory ailment.

Asked about Ali's condition, the source said: "It's extraordinarily grave. It's a matter of hours."