Masked offenders armed with hammers have robbed two service stations in the northern suburbs.

Police said that four masked offenders smashed their way into a service station on the corner of Wood Street and Dalton Road, Thomastown about 4.45am on January 10.

The attendant managed to lock himself in a back room.

The offenders stole a significant amount of cigarettes before leaving in a silver SUV.

Around the same time, two masked offenders, armed with hammers and a knife, entered a store on the corner of Dalton Road and Keon Parade, Keon Park.

The pair jumped the counter and demanded the store attendant open the register.

Four other offenders then entered the store and stole cigarettes.

The attendant was struck to the face but was not injured.

The offenders left the store in what is believed to be a black sedan.

Police said both groups of offenders were perceived to be of African appearance.

They are working to establish whether the two incidents are linked and are appealing for anyone with information to contact Crime Stoppers on 1800 333 000.