A massive blaze at a Keilor East factory which took 60 firefighters three hours to bring under control is being treated as suspicious.

Fire crews were called about 4am when a fire broke out at Metro Drains in Thor Court, sending large plumes of smoke across the Western Ring Road and into the Melbourne Airport flight path.

A dozen trucks and excavators were destroyed in the suspicious blaze in Keilor East. Credit:Channel Nine

The damage bill is believed to be more than $5 million.

Issues with accessing water meant crews had to pump water from more than 500 metres away, losing crucial pressure to help extinguish the blaze.