Greta Gerwig has signed up for the cast of Jackie alongside Natalie Portman and Peter Sarsgaard.

Gerwig will portray an aide to Portman's Jacqueline Kennedy as she copes with the first four days following US president John F Kennedy's November 1963 assassination in Dallas, Digital Spy reports citing Variety.

Sarsgaard joined the film earlier this week as Robert Kennedy, with Portman long established as starring.

No's Pablo Larraín is set to direct, with Portman's Black Swan collaborator Darren Aronofsky producing.

Aronofsky was previously developing Jackie as a directorial project with former partner Rachel Weisz due to star, but it was shelved when the couple split.

Production on Jackie is expected to start in late 2015.