NEW YORK -- Residential mortgage delinquencies and defaults are becoming more common among borrowers in the category just above subprime, American International Group said on Thursday.

AIG, 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 gave a gloomy assessment of the market in a presentation to investors and analysts.

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It said delinquency rates for first mortgages had risen to 3.98 percent in June from 3.56 percent in April and a low of 3.08 percent in July 2005. First mortgages represent 90 percent of AIG's domestic mortgage business.

AIG divided its mortgage portfolio into three categories: subprime, for borrowers with credit scores below 620; "non-prime," for borrowers with credit scores between 620 and 659; and prime, for borrowers with credit ratings above 660.

As of June 30, AIG's finance arm, which originates first and second mortgages, recorded delinquencies of 3.68 percent in subprime, 2.13 percent in non-prime, and 0.81 percent in prime.

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