Katy Perry and Taylor Swift are "very competitive."
The 'Roar' hitmaker, 29, reportedly instantly knew that the 24-year-old singer, who also once dated her ex-boyfriend John Mayer, was talking about her when she revealed this week that she has penned a new track, 'Bad Blood,' about another famous female popstar that she considers one of her "straight-up enemies" after the rival, who she did not name, tried to "sabotage" her entire 'Red' world tour.
A source close to Katy, who posted a mysterious message about an unnamed mean girl on Twitter, saying "Watch out for the Regina George in sheep's clothing," one day after Taylor's interview was published, told Us Weekly magazine: "Katy knows Taylor's song is about her, which is why she tweeted that."
The insider added: "[They can be] very competitive."
The duo were previously thought to have been amicable, but never became real friends.
In her interview, Taylor also suggested there was a "personal" element to the conflict and claimed the other singer "tried to sabotage an entire arena tour" by hiring her dancers.
And I'm surprisingly non-confrontational - you would not believe how much I hate conflict. So now I have to avoid her. It's awkward, and I don't like it.
But the source close to Katy said: "They were loyal to Katy when it came time to launch the 'Prism' tour. She didn't steal them. Taylor just employed them when they weren't working for Katy."
Many fans thought that the 'I Knew You Were Trouble' hitmaker was talking about Miley Cyrus in the interview, but a source close to the 'Wrecking Ball' singer said she "didn't steal [Taylor's] tour people. She's not the focus on the song."
But the pal admitted: "Miley is competitive with Taylor."