A suspicious fire has damaged five vehicles at a garage in Sydney's southwest.

Police and firefighters were called to a unit block on Coffee Way, Claymore, about 2.30am (AEDT) on Saturday following reports of a fire in the garage.

Three vehicles were destroyed and two others extensively damaged before firefighters could extinguish the blaze.

Witnesses have reported seeing two men running from the area a short time after the fire broke out, police said.

A crime scene was set up while detectives continue their investigation into the blaze, which is being treated as suspicious.

Anyone with information about the incident is urged to contact Crime Stoppers on 1800 333 000.