Standard & Poor's Rating Service said Friday that it placed Citigroup's counterparty rating of AA-/A-1+ on CreditWatch.

It also placed Citigroup's long term AA counterparty credit ratings for the bank's subsidiaries on negative watch.

Citigroup's announced loss of $5.1 billion for the first quarter reflects not only further marks to market of securities, but large additions to reserves in anticipation of further deterioration in its loan portfolio, the ratings service said.

Earnings for the medium term may be affected in expectation of additional problems.