Moody’s Investors Service sent warnings Wednesday to Citigroup Inc. in New York and HSBC Holdings in London that it might downgrade their structured investment vehicles as the rating company reviews $33 billion worth of the debt.

SIV “debt ratings continue to be vulnerable to the unprecedented large and sustained declines in portfolio value combined with a prolonged inability to refinance maturing debt,” Moody’s said.

The company is reviewing three Citigroup and two HSBC structured investment vehicles.