Ratings agency Moody's on Monday downgraded the debt ratings of collapsed New York-based Signature Bank (SBNY.O) deep into junk territory and placed the ratings of six other US banks under review for a downgrade.

Moody's, which rated Signature Bank's subordinate debt 'C', said it was also withdrawing future ratings for the collapsed bank.

The banks placed under review for downgrade are First Republic Bank (FRC.N), Zions Bancorporation (ZION.O), Western Alliance Bancorp (WAL.N), Comerica Inc (CMA.N), UMB Financial Corp and Intrust Financial Corporation, Moody's said.

State regulators closed Signature Bank on Sunday, the third largest failure in U.S. banking history, two days after authorities shuttered Silicon Valley Bank (SIVB.O) in a collapse that stranded billions in deposits.