NEW YORK—Fitch Ratings downgraded the financial-strength ratings of American International Group Inc.'s property/casualty subsidiaries Thursday, including its Chartis Inc. units, although other rating agencies reaffirmed their ratings on the units.

The move followed AIG's Wednesday announcement that it expects to boost the reserves of Chartis' property/casualty subsidiaries by $4.6 billion, which it said will result in a $4.1 billion reserve charge when its posts its fourth-quarter results Feb. 24.

Fitch downgraded the subsidiaries to “A” from “A+,” although it affirmed AIG's issuer default rating and debt ratings as well is its life insurance unit's financial-strength ratings.

“We just didn't think their reserve stability and the underwriting profitability really supported that (A+) rating any longer,” said Mark Rouck, an analyst at Fitch. “We think that the profitability and the reserves stability is much more likely to be supportive” of the “A” rating.

Mr. Rouck also cited AIG's $2.3 billion pretax loss reserve strengthening for commercial insurance that it reported for 2009's fourth quarter. “We view the development that AIG's taken in the last couple of years as an outlier relative to some of its peer companies, as well as the overall property/casualty industry,” he said.

Mr. Rouck said the 2011 reserve charge “in some respects points out the difficulty of trying establish reserves” for long-duration excess casualty lines, for which AIG had held large market shares. There are a lot of “challenges with trying to accurately establish reserves for those businesses,” he said.

However, other rating agencies affirmed Chartis' ratings.

Oldwick, N.J.-based A.M. Best Co. Inc. affirmed Chartis U.S. Insurance Group's "A” (excellent) financial-strength rating and its “a” issuer credit rating on Wednesday, stating that it views the charges “as being within its previous estimate of the group's reserve deficiency.”

New York-based Standard & Poor's Corp. said Wednesday that its ratings on AIG and its insurance subsidiaries, including its “A+” rating of Chartis, are unaffected by the reserve charge, noting that its consolidated capital “will remain strong and appropriate for the ratings.”

New York-based Moody's Investors Services said Wednesday that its Chartis “A1” and AIG ratings remain unchanged, “although this announcement underscores the company's challenges in estimating claims costs for long-tail casualty lines.”