John Travolta is "proud" of the way his Grease co-star Olivia Newton-John has been coping with her cancer battle.

The singer and actress is currently battling stage four cancer - her third encounter with the disease. And speaking to Entertainment Tonight at The Fanatic premiere in Hollywood on Thursday, Travolta shared he was "proud" of his longtime friend.

"She looks incredible," the 65-year-old marvelled. "She doesn't look any different than (she did) years ago, and I'm very proud of her."

"I'm very happy about Olivia," he smiled.

The Xanadu star, 70, first battled the disease in 1992, when she underwent a partial mastectomy and reconstruction, and again when it returned in her shoulder in 2013.

However, she recently told the publication she's "really well and really healthy", adding: "It was a challenging year because I broke my sacrum (a bone in the lower back) and I had to learn to walk again and everything.

"But I am strong and I am back and I'm feeling good and loving every minute... I just want everyone to know, I'm here (and) I'm doing great."