Ozzy Osbourne has been left stunned by the success of Black Sabbath's long-awaited comeback album, 13, because he didn't expect it to be a big hit.

The new release has already topped download charts in 23 countries and is set to debut at number one around the world, and frontman Ozzy still can't believe the old rockers have another hit on their hands.

He says, "It's really good but I never thought it was gonna be a success... There's no airplay on it."

And the frontman accepts the band don't make it easy for radio bosses to play their tracks: "We don't write four or five or three-minute songs; we just play until it's time to stop."

The Paranoid singer has one major goal in mind - he wants to top the U.S. album chart.

He adds, "It'd be great if it got to number one on the Billboard chart."