Moody's Investors Service has warned that AAA ratings of four leading bond insurers could be downgraded after the agency re-evaluated the companies' exposure to potential subprime mortgage losses.

The AAA ratings of Financial Guaranty Insurance Company and XL Capital Assurance, a unit of Security Capital Assurance, have been placed on review for possible downgrade, Moody's said.

The AAA ratings of MBIA and CIFG Guaranty were affirmed, but the rating outlooks changed to negative.

While the AAA ratings of Ambac, Assured Guaranty and Financial Security Assurance have been affirmed with a stable outlook.