NEW YORK (Reuters) -- Residential mortgage delinquencies and defaults are becoming more common among borrowers in the category just above subprime, American International Group said Thursday. AIG (Charts, Fortune 500), the world's largest insurer and one of the biggest mortgage lenders, said total delinquencies in its $25.9 billion mortgage insurance portfolio were 2.5 percent. It said 10.8 percent of subprime mortgages were 60 days overdue, compared with 4.6 percent in the category with credit scores just above subprime, indicating that the threat to the mortgage market may be spreading. While maintaining that it is "comfortable" with its mortgage exposure, AIG...