Harrison Ford was the toughest actor to bring on board for The Expendables 3, Sylvester Stallone has revealed.

The Rocky star admitted that he spent hours on the phone on his birthday, trying to convince the 72-year-old actor to join the cast as CIA head Max Drummer in the third instalment of the action franchise.

"It's very time-consuming. It takes a long time to convince some people. You're calling them for favours on your birthday and on the phone for four hours," he said. "You're like, 'It's my birthday. Do me a favour!' You do a lot of begging, it's wild."

He revealed Mel Gibson - who plays The Expendables' main rival Conrad Stonebanks - proved slightly easier to convince. Other newcomers included Wesley Snipes, Antonio Banderas and Kellan Lutz.

Sly continued: "Originally I wanted Mel to direct. He wanted to know about the part. Everything is so personal with him and he said, 'I wouldn't mind playing a villain'. I said, 'Sold!'."

The 68-year-old actor, who also co-wrote and co-produced the film, admitted there was inevitably some friendly rivalry amongst the 17 stars.

"Everyone here has healthy egos and athletic ability - some are very good actors and you're going to be competitive," he said. "No one wants to be the one sitting there, looking like a piece of chewed string at the end of scene, so you do your best."

Sly also revealed that he plans to make another two or three movies in the series, although he'd be very keen to go up to Expendables 10.

"Yep, you heard it here first! It all depends on this one yet, but I can see maybe two more. We'd like to continue because it's an on-going experiment," he said.

:: The Expendables 3 is in cinemas now.