NEW YORK—The A+ financial strength and counterparty credit ratings outlook of a number of American International Group Inc.'s property/casualty subsidiaries and other units was changed to negative from "developing" by Standard & Poor's Corp. on Wednesday.

AIG is not expected to sell its core property/casualty operations under its restructuring plan, the New York-based rating agency said.

"The ratings on these companies...reflect our view of the likelihood of increased pressure on the performance of that business," S&P credit analyst Rodney A. Clark said.

S&P said it revised its outlook on the entire U.S. commercial lines property/casualty sector to negative because of weakening pricing and adverse investment markets in August.

"We believe AIG is particularly susceptible to these broader market trends, given its somewhat weakened position," Mr. Clark said.

"Although at this point we have not seen clear evidence of long-term damage to AIG's franchise, there have been wide reports that competitors are actively pursuing AIG's accounts and key underwriting personnel, which could pressure operating performance over time given the...market conditions the industry is facing."

The parent company's ratings remain under review with a negative outlook, and the ratings status of most of AIG's life insurance operations, which S&P said AIG is expected to sell, continues as developing.

An AIG spokesman could not be reached.