'It's time' for Taylor Swift to end our 'petty' feud - Katy Perry

US pop singer Katy Perry has finally spoken out about the long-term hostility she and Taylor Swift have for one another, telling talk show host James Corden that Swift started the "petty sh*t" between them.

In an appearance on Corden's famous segment Carpool Karaoke, Perry sang her new song 'Swish Swish' - believed to be a diss track against Swift - before going into how the high-profile beef started.

The 'Firework' hitmaker explained that the rivalry came about as a result of a contract dispute involving backing dancers.

Perry, who claims she's close to the dancers, told them they could go on tour with Swift - but she urged them to put a clause in their contract that would allow them to leave Swift's tour and join her's once she started performing herself.

However Swift and her management took offence to their decision to leave mid-world tour, allegedly firing them on the spot rather than letting them work out the 30 days they were obligated to as stated in their contract.

Perry says she immediately attempted to contact Swift when she heard what had happened.

"I tried to talk to her about it and she wouldn't speak to me," Perry told Corden.

"I do the right thing any time it feels like a fumble. It was a full shutdown, and then she writes a song about me, and I'm like, 'Cool, cool, cool, that's how you wanna deal with it?'

"Karma," she said with a clap, presumably referencing the fall from grace Swift experienced when it was revealed she lied about not giving rapper Kanye West permission to reference her in his song 'Famous'.

The song of Swift's that Perry was referring to was her 2015 track 'Bad Blood', which is widely recognised to be about her.

"What I wanna say is that I'm ready for that BS to be done. Now, there is the law of cause and effect - if you do something, there's gonna be a reaction. And trust me daddy, there's gonna be a reaction," she said.

But when Corden urged her to "draw a line under the beef", Perry agreed, saying she was an advocate for women empowering each other and that they can "heal the world" if they join forces.

Perry said should Swift send her a text saying the feud is over, she would "100 percent" rescind any hard feelings towards her.

"There is a situation. Honestly, it's really like she started it and it's time for her to finish it," she said.