Amy Winehouse brands Pete Doherty's music "s*it"

Amy Winehouse has branded pal Pete Doherty's music "s*it".

"Amy has thrown back every song Pete has written for her," says a source close to the Back to Black singer. "She keeps telling him it's rubbish -- and worse -- and she'll only perform if the song is great.

"When she's in the studio she can be ruthless and doesn't care about hurting people.

"Pete's really struggling to get it together. It hasn't clicked yet."

Amy and Pete -- who was recently released from jail after serving 29 days of a 14-week sentence for breaking bail conditions -- attempted another duet at the Jazz After Dark venue in London's Soho Wednesday, but didn't arrive at the the club until 2am, by which time it was already closed.

Meanwhile, the 24-year-old Grammy winner is said to be in talks to perform for the girlfriend of Russian billionaire and Chelsea football club owner Roman Abramovich.

A source said: "That deal hasn't been finalised yet - it all depends on whether they'll give her a visa."