Miley Cyrus and Liam Hemsworth are back on after the engaged couple were spotted kissing on May 10. Credit: Jason Merritt/Getty Images

"Can't Be Tamed" singer Miley Cyrus can't let go either. She and Liam Hemsworth, 23 -- who hit a rough patch this spring and stopped planning their wedding -- got cozy at a party at her L.A. home May 10.

"She was kissing him," says a witness. "They were definitely back on."

Her engagement ring has reappeared too. After flaunting her bare finger in March and April, Cyrus, 20, rocked the 3.5-carat Neil Lane sparkler at the May 15 Maxim Hot 100 party at Create in Hollywood.

Still, friends doubt that the couple will last. "They break up, get back together, repeat," says one pal. "They will eventually burn out."

Hemsworth's actor siblings have also tried to put a stop to his relationship. A source told Us that Chris and Luke staged an intervention with their younger brother in April.

The source said, "They want him to end the romance for good."

Cyrus and Hemsworth became engaged last June after falling in love on the set of their 2009 film, The Last Song.