NEW YORK, April 18 (Reuters) - The rating for Arizona's Verde $1.13 billion of gas bonds could be cut again because Citigroup's (C.N: Quote, Profile, Research) big first-quarter loss has imperiled the bank's rating, Standard & Poor's Ratings Services said on Friday.

Citigroup guarantees the obligations of its commodity arm to provide Salt Verde with gas, S&P said.

Citigroup could suffer more write-downs in its loan portfolio, the credit agency said in a statement, in another sign of the havoc wreaked by the subprime mortgage problems. Citi now is rated "AA-" by S&P but on Friday the credit agency warned it might cut the bank's rating.

In January, S&P cut its rating on the Salt Verde debt to "AA-" from "AA." (Reporting by Joan Gralla; Editing by Leslie Adler)