(Reuters) - S&P Global downgraded its outlooks on five regional U.S. banks to "negative" from "stable" due to their commercial real estate (CRE) exposures, the ratings agency said on Tuesday.

It downgraded First Commonwealth (NYSE:FCF) Financial, M&T Bank (NYSE:MTB), Synovus (NYSE:SNV) Financial, Trustmark (NASDAQ:TRMK) and Valley National Bancorp (NASDAQ:VLY).

"The negative outlook revisions reflect the possibility that stress in CRE markets may hurt the asset quality and performance of the five banks, which have some of the highest exposures to CRE loans among banks we rate," S&P said.

The most recent downgrades bring the total negative outlooks on nine U.S. banks, or 18% of those that S&P rates, it said, adding most of those "relate, at least in part to sizable CRE exposures".

CRE exposures of regional banks were heavily scrutinized this year since New York Community Bancorp (NYSE:NYCB) flagged a surprise loss and slashed its dividend citing loan loss provisions on CRE loans, which triggered a sell-off in U.S. regional banking shares.

Investors and analysts alike have been worried that higher borrowing costs and low occupancy rates for office spaces could exacerbate the stress on lenders exposed to potential defaults by borrowers in the CRE sector.

However, S&P said it maintained a 'stable' outlook on F.N.B Corp, "since we see a somewhat lower probability of material deterioration in its asset quality and performance".