How to ensure the authenticity of a company job offer asking for money upfront

A UAE-based company for oil & gas sends a letter of employment but asks for almost $ 5000 for VISA and other expenses collected by an immigration services agency upfront by the future employee.

Promises in writing to reimburse, but doesn't give access to a bank account. Everything has to be pre-financed. I already paid € 1700!

Is this serious?