The force said it was working with the Metropolitan Police to find Elmi.
Det Sgt Matthew French, of Buckinghamshire’s Priority Crime Team, said: “If you see Elmi, do not approach him and call 999 instead.”
A Ministry of Justice spokesperson said: “Absconding is a serious criminal offence, and any defendant who commits this crime could face longer behind bars.
“The police are working urgently to recapture this individual, and we urge anyone with information to go to the police.”
HMP Spring Hill, which has more than 300 inmates, is classed as Category D.
According to the Prison Service, that means it has “minimal security and allow(s) eligible prisoners to spend most of their day away from the prison on licence to carry out work, education or for other resettlement purposes”.
Anyone who can help should call 101 quoting reference number 43250653119.