Financial fair play (FFP) needs to be more "robust" and the rules are "weak" in certain areas, Uefa president Aleksander Ceferin has told BBC Sport.
FFP 'break-even' rules require clubs to balance spending with their revenue.
German news magazine Der Spiegel has claimed Manchester City and Paris St-Germain overvalued sponsorship deals to help meet the rules.
Uefa said it would reopen FFP inquiries "on a case-by-case basis" if there was evidence of "abuse".
City have said they would not comment on Der Spiegel's claims, apart from to describe them as an "organised and clear" attempt to damage their reputation.
PSG said it "has always acted in full compliance with the laws and regulations enacted by sports institutions" and it "denies the allegations".
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Ceferin said: "I don't want to speak about Man City or PSG but for any club the rules have to be strong and clear. We will act by the book, by the regulations.
"We know that we have to modernise. We know we have to check the rules and regulations all the time. We know that the situation in the football market is changing all the time. So that's also part of our thinking for the future - do we have to do something about the regulations to be more robust? Yes."
Asked if Uefa could use sporting sanctions against clubs that break FFP rules, such as barring them from the Champions League, Ceferin added: "There are many things we are talking about - also sporting sanctions and everything else.
"It's the start of the debate. It's a bit premature to speak about it but we acknowledge the rules might be weak in certain points. Also laws in certain countries are changing all the time [and] adopting to modern times."
In its reporting based on leaked documents, Der Spiegel also said the clubs negotiated with Fifa president Gianni Infantino, who was then general secretary of Uefa, to agree reduced punishments on FFP breaches.
Uefa found City had breached FFP rules in 2014 and the two parties reached a settlement, with City paying a £49m fine - £32m of which was suspended - while their Champions League squad was reduced for 2014-15.
"Our independent bodies will check it," said Ceferin. "I know they will. But we also know we have to keep our credibility. Nobody cares if it happened four years ago when the leadership was different - it is about the organisation."