Before the Bell-MBIA, Ambac shares drop as rating cut

(Adds Ambac shares falling)

NEW YORK, June 20 (Reuters) - Shares of MBIA Inc (MBI.N) and Ambac Financial Group Inc (ABK.N) fell before the opening bell on Friday after ratings agency Moody's Investors Service stripped the top-notch "Aaa" rating on the insurance arms of both companies late on Thursday.

Ambac shares were down 6.4 percent at $1.90 and MBIA shares fell 10.4 percent to $5.78. (Reporting by Ellis Mnyandu; Editing by Chizu Nomiyama)