U.S. stocks were mixed Wednesday afternoon after Moody's said it may be forced to downgrade the credit ratings on MBIA Inc and Ambac Financial.

The S&P 500 also briefly turned negative.

As of 2:33 p.m. EDT, the Dow Jones Industrial Average lost 14.08 points, or 0.11 percent to 12388.04, the Standard & Poor's 500 index rose 0.56 points, or 0.04 percent, to 1377.93 and the Nasdaq Composite Index picked up 20.76 points, or 0.84 percent, to 2500.55.

Both MBIA and Ambac Financial fell sharply after Moody's said it will most likely have to downgrade the companies' coveted AAA credit ratings.

The ratings agency cited the companies' diminished new business prospects and financial flexibility, along with the chances they may have to take higher losses from their insurance portfolios. MBIA shares were down $1.14 to $5.57 and Ambac fell 55 cents to $2.45.

United Airlines soared nearly 11 percent to lead the Nasdaq 100 after the airline released plans to cut back domestic capacity and retire less-fuel efficient jets. Shares were recently up 89 cents or 10.4 percent to $9.42.