Have to agree with porsche, the French /u/ does not exist in English. Physiologically, there is no lower jaw movement in the French /u/, which is uncharacteristic of most English /u/ pronunciations. One of the easiest ways to tell a Francophone from an Anglophone is /u/ production. Years spent teaching English to the French has shown that native vowel sounds are the biggest hurdles in proper pronunciation. They determine so much in English, like stress and prosody, that changing them is primordial to movement across the two languages.