No similar sound in English, but the same as the vowel in French rue. To produce it, say the German long i, then round the lips as for long u. Do not allow your tongue to move toward the back of your mouth as you round your lips. Note the difference between the long rounded back vowel u, and the long rounded front vowel ü.

Note: This is one of the most difficult German sounds for American speakers of English to produce accurately. Practice it until you have internalized its characteristics.