Global rating agency Fitch today upgraded the long term foreign currency issuer default rating (IDR) of ICICI Bank from non-investment to investment grade following the recent upgrade of the sovereign rating. It also upgraded the bank's short term currency rating to fair quality grade.The IDR has been upgraded to 'BBB-" from 'BB+' while the short term foreign currency rating has been upgraded to 'B' from 'F3'. The long term rating outlook is stable, the rating agency said in a release.Meanwhile, the rating on the bank's outstanding Rs 50 crore bonds have also been upgraded to investment grade (BBB-) from earlier non-investment grade (BB+), it said.ICICI's foreign currency ratings continue to reflect the bank's improved financial condition, the rating agency said. However, it said, the key challenges is to maintain asset quality following the rapid loan growth since FY03 with the bulk of loans staring season during the environment of rising interest rate, which could affect borrowers' repayment capacity.