"An interjection used to indicate an affirmative answer." So defines the Random House Dictionary. It also defines "uh-uh" as the opposite of "uh-huh". Webster's Second defines it in almost the same words but does not define "uh-uh". The Oxford English Dictionary defines neither word but does define "uh". American Heritage simply ignores the whole thing.
Word Ways: Vol. 18
, Article 15.
Available at: http://digitalcommons.butler.edu/wordways/vol18/iss4/15