(Reuters) - Ratings agency S&P Global on Tuesday downgraded five regional U.S. banks to due to their commercial real estate (CRE) exposures, in a move likely to reignite investor concerns about the health of the sector.

The ratings agency downgraded First Commonwealth Financial ( FCF ), M&T Bank ( MTB ), Synovus Financial ( SNV ), Trustmark and Valley National Bancorp ( VLY ) to "negative" from "stable," it said.

"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.

Representatives for the banks did not immediately respond to request for comments outside business hours.

Investor concerns over regional banks' CRE exposure intensified this year after New York Community Bancorp flagged a surprise quarterly loss citing provisions on soured CRE loans, which triggered a sell-off in U.S. regional banking shares. The bank has sold assets to shore up its balance sheet.

Investors and analysts have been worried that higher borrowing costs and lingering low occupancy rates for office spaces in the aftermath of the COVID-19 pandemic could result in more lenders taking losses as borrowers default on loans.

Tuesday's downgrades come a year after the collapse of Silicon Valley Bank and Signature Bank, which heightened investor sensitivity about the health of U.S. regional banks.

In addition to CRE exposure, the sector is also facing challenges from the rising cost of retaining deposits amid high interest rates.

As of Tuesday, S&P had negative outlooks on nine U.S. banks, or 18% of those it rates, it said, adding most of those ratings "relate, at least in part to sizable CRE exposures." The company rates a range of banks of varying sizes.