Fire crews are battling an out of control blaze at the former Maribyrnong Defence Site.

Dozens of firefighters are working to contain a factory fire in a disused building off Cordite Road.

Emergency services were alerted to the fire just before 1pm when a grass fire in the area spread to the building.

Paramedics called to an address in Maribyrnong at 1.03pm assessed two people at the scene for smoke inhalation.

Water-bombing helicopters joined the battle to contain the blaze, along with more than 20 MFB fire appliances.

Firefighters remain unsure of what is burning in the building and are awaiting advice from the department of defence before launching an internal attack.

No threat has been identified to the community, but advice has been issued for Avondale Heights, Maidstone, Maribyrnong residents to stay informed and monitor conditions.

Residents are urged to keep nearby roads roads clear for emergency services, move indoors, close doors and windows, and shut down air conditioning.

People with existing health conditions including asthma should follow the treatment plan advised by the doctor.