Etreme  Assault u.s cheat codes=type "oh dear" (small letters) in
the main menu.

After that you will have various cheats.
Single player only.

alt1=full ammo
alt2=upgrade current weapon
alt3=full energy
alt4=indestructible
alt5=wow
alt6=mission compleate
alt7=deactivate enemies
alt8=heli mode
alt9=tank mode
alt0=speeeed!

Type EMERTXE at the main menu to activate Assault Mode:

Alt-U Fly a UFO
Alt-F ?
Alt-L ?
Alt-T ?