A 13-year-old boy has been arrested but three others are still on the run after allegedly assaulting a Darwin service station attendant, NT Police said.

NT government crime marketing manager Sita Bacher said in a statement that just before 5am Tuesday, four male youths approached a man outside of a service station along Ross Smith Avenue, demanding keys to the shop. The only service station in Fannie Bay is the Puma on the corner of Dick Ward Drive.

She said the youths allegedly assaulted the victim, before getting into the store and making off with an unspecified amount of cash. Police did not give details of the seriousness of the assault or mention any injuries suffered by the service station attendant.

Officers are sill looking for the other three but a 13-year-old boy was arrested and was expected to be charged on Tuesday Ms Bacher said. Police did not specify what charges were likely.