Lewis Hamilton ended Sunday's Belgian Grand Prix wondering if Ferrari had used technical "tricks" for victory.
"They've got a few trick things going on in the car," Hamilton alleged.
"They're able to deploy more than us, somehow, from turn one to Eau Rouge, and then it carries on down the straight. I'm not really sure how."
Earlier this year, suspicions arose about the legality of Ferrari's power unit, but the FIA looked into it and concluded that everything was above board.
When asked about Hamilton's latest 'trick' comment, F1 race director Charlie Whiting said on Sunday: "It amuses me, because we know a lot about Ferrari's car and Lewis does not.
"The FIA knows what is at stake and we're happy."
Journalists wondered if Hamilton was making an accusation of cheating, but the Briton backed down from that claim.
"No, we all have trick things on our cars. Trick is just a word for something special, I guess," said the world championship leader.
"I'm not saying that there's anything illegal. I don't mean anything to it so please don't twist my words and say that I said they're doing anything illegal because they're not."
Hamilton's Mercedes boss Toto Wolff, however, backed his driver's suspicions.
"When you're beaten on the track, then you look at yourself and the others. And if you can't find an explanation, you get dirty thoughts," he said.
"But I have great faith in the FIA that they have everything under control."
Sunday's result leaves Hamilton 17 points ahead of Vettel at the top of the driver rankings.