A judge voiced concern about a $298m (£190m) settlement between Barclays and the US Justice Department, labelling it "a sweetheart deal".
The British bank agreed to pay the sum to settle criminal charges that it violated US sanctions in dealings with Cuba, Iran, Libya, Sudan and Burma.
But the US district judge who must approve the settlement said he was worried about its implication.
"This is a sweetheart deal," said Judge Emmet Sullivan. "This concerns the court, and that should concern the government too."
Barclays was charged with violating the International Emergency Economic Powers Act and Trading with the Enemy Act, according to court documents.
Meanwhile, another US judge has refused to approve the Securities and Exchange Commission's $75m settlement with Citigroup.
Judge Ellen Huvelle asked for more information before approving the proposed deal, which was struck over allegations that the bank misled investors by failing to disclose billions of dollars of subprime mortgages.
"It's a home run for the good guys," said Richard Greenfield, a lawyer representing one Citigroup shareholder. "The bottom line is that the SEC put virtually nothing forward to justify the settlement. Citigroup shareholders have already been victimised, and we don't want that to happen a second time."