"It's not Skrulls, I've been clear about that," he said. "Beyond that, we want to hide it. The identity of the alien race is not impactful."
Reports that Captain America's nemesis Red Skull (Hugo Weaving) will be part of The Avengers have also been debunked.
Robert Downey Jr, Chris Evans, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Samuel L Jackson and Tom Hiddleston star in The Avengers, which arrives in theatres on April 27 in the UK and on May 4 in the US.
Watch Tom Hiddleston talk to Digital Spy about The Avengers below: