BNP Paribas chief executive Jean-Laurent Bonnafe acknowledged at Wednesday's annual meeting that a fine could be "very significantly higher" than the $1.1 billion it has set aside to settle charges it did business in Iran and other countries under US sanction.
"The situation is obviously difficult," said Bonnafe.
Officials at Credit Suisse last week said the bank was working to resolve a criminal probe on enabling US taxpayers to evade taxes through secret bank accounts.
Switzerland's second largest bank has set aside $810 million to resolve the case, but latest reports have said the bank could end up paying two and a half times that amount.
Credit Suisse also faces an investigation in Switzerland over potential stock exchange trading violations.
The crackdown on the two European giants follows major federal settlements with JPMorgan Chase, Bank of America and other large US banks over myriad violations.
"There is no such thing as 'too big to jail,'" Attorney General Eric Holder said earlier this month.
"When laws indeed appear to have been broken -- and the evidence supports the allegations -- a company's size will never be a shield from prosecution or penalty," he said.
No guilty pleas for US banks
But in similar cases, US banks have been fined heavily but not forced to plea guilty.
For example, the Justice Department won a record $13 billion settlement from JPMorgan Chase last year to settle myriad charges over fraudulently labelled mortgage-backed securities.
In January, JPMorgan agreed to pay $2.6 billion to settle criminal charges its lax oversight enabled the multibillion-dollar Ponzi scheme of Bernard Madoff.
The bank acknowledged a lengthy statement of facts outlining the wrongdoing and signed a deferred prosecution agreement.
But in neither case did JPMorgan have to admit the charges.
Moreover, the Justice Department has not criminally prosecuted any senior bank officials in the mortgage securities debacle, a decision that US Judge Jed Rakoff suggested could be "one of the more egregious failures of the criminal justice system in many years."
One reason prosecutors have been loath to force guilty pleas until now is the fear of that a conviction could trigger dire consequences, such as the withdrawal of a bank's charter to operate, endangering jobs and threatening the economy.
"You take into consideration all factors," US Attorney Preet Bharara said in January, noting "collateral consequences" to pursuing a criminal case against a big financial institution.
But in March Bharara, referring to a case against "a significant financial institution", said the department would test claims by prosecuted banks that a guilty plea risked "potential devastation".
In the Credit Suisse and BNP cases, New York state's top financial regulator Benjamin Lawsky has told both that they could keep their charters if they plead guilty and agree to significant penalties, according to a person familiar with the matter.
Samuel Buell, a former US prosecutor who teaches criminal law at Duke University, said the two cases reflect Justice's shifting view of the "feasibility of doing guilty pleas."
But Brandon Garrett, a professor at the University of Virginia School of Law, said they raise concerns that US and foreign banks are being treated differently.
"It may be that foreign companies aren't prosecuted unless it's a bigger case, and it may be that foreign companies care less about the consequences of a plea in some cases, but that's a real concern," Garrett said.
"But I do think that going forward, if convictions can become more routine that's a good thing.
"Normally, people think that if serious crimes are committed, the appropriate result is conviction."