Homes are under threat and a major highway is closed as grass fires burn north of Melbourne.


Around 2pm, 31 trucks were fighting a blaze at Epping and other grass fires along the Hume Freeway. One fire had come within 31 metres of a built-up area.


The Hume Freeway remains closed in both directions between Cooper Street and Donnybrook Road.


A wind change is expected to change conditions at 4pm.


"It’s hard to know what sort of impact it will have on the fires. That’s why we’re urgently working to get them under control," a CFA spokeswoman said.


Fairfax radio has been told the fires are being treated as suspicious.


There are concerns they could be the work of a fire bug.


A watch and act warning has been issued for the towns and suburbs of Kalkallo, Epping, Donnybrook, Craigieburn, Campbellfield, Roxburgh Park, Somerton and Lalor.


"There are multiple fires along the Hume highway stretching from Beveridge towards the Metropolitan Ring Road," the CFA said.


"These grassfire are currently not yet under control."


Metro has suspended the Craigieburn train line from Essendon to Craigieburn because of the fires. Replacement buses are on order.