Mark Ruffalo has apologised to Marvel after sharing some photos from the set of the Avengers sequel.

The 46-year-old, who reprises his role as Bruce Banner and the Hulk in the follow-up, posted some behind-the-scenes images on his Instagram and Tumblr accounts.

"I'm sorry @marvel. I will be better. I'm like your black sheep Avenger," he tweeted.

One photo showed an elusive red-headed Scarlett Johansson (Natasha Romanoff/the Black Widow) in a character-branded gown, with the caption: "Almost got her!!!"

Another picture saw his reunion with Robert Downey Jr aka Tony Stark/Iron Man, which he labelled: "robertdowneyjr Bros".

Robert then followed up Mark's posts with an image of his own - six chairs featuring the names of the stars - himself, Scarlett, Mark, Chris Evans, Jeremy Renner and Chris Hemsworth.

Joss Whedon's sequel, Avengers: Age Of Ultron, began shooting in early April and will be released in May 2015. It will also star Aaron Taylor-Johnson, Elizabeth Olsen and James Spader.