David Koepp thinks it was a mistake to include aliens in 'Indiana Jones and the Kingdom of the Crystal Skull'.

The 59-year-old screenwriter penned the script for the 2008 movie – which marked the return of Harrison Ford's legendary adventurer after nearly two decades – but was unhappy with the inclusion of extraterrestrials in the film and tried to persuade director Steven Spielberg and franchise creator George Lucas that it wasn't a good idea.

Speaking on the Script Apart podcast, David explained: "I was never happy with the idea (of including alien-like creatures in 'Kingdom of the Crystal Skull'). When I came on, I tried to convince (Steven Spielberg and George Lucas) to change it – I had this other idea. They didn't want to change it."

The film received a mixed response from audiences and Koepp believes that the aliens played a big part in the lukewarm response.

He said: "I'm not saying mine would've been better. But I think that a lot of the pushback that movie got, in a larger sense aside from little things people might not have liked – that were too silly or whatever – the larger one was that (fans said) 'We don't feel like aliens should've been in an 'Indiana Jones' movie'. Fair enough, in retrospect, you're probably right."

Koepp has not been involved in the creation of the script for the upcoming 'Indiana Jones 5' and explained that he wanted new director James Mangold to have the chance to craft his own story after taking over from Spielberg.

He explained: "James deserves a chance to take his shot at it.

"I've done several versions with Steven. And when Steven left, it seemed like the right time to let Jim have his own take on it and have his own person or himself write it."