Muhammad Ali's talent, charisma and strong principles were responsible for raising boxing's popularity across the world, according to former US president Bill Clinton.

Clinton also credited Ali, who turned 70 on Tuesday, with helping pave the way for Barack Obama to become the country's first black president.

"He made millions of people believe. He was something unique," said Clinton.

Three-time world heavyweight champion Ali won 56 bouts over a 21-year career.