Reigning Formula One constructors' champion Mercedes took the center stage at the preseason testing in Bahrain on Thursday after introducing a radically revised car.
The new-look W13 car featured dramatically narrower sidepods, with Formula One's motorsport managing director, Ross Brawn, saying the concept was impressive and a "very extreme interpretation of the regulation" that had not been anticipated.
"Inevitably, there's going to be a lot of debate about their interpretation. That's what happens with new regulations," he told F1 TV.
"However hard you try to close off all the options, and believe me we closed off hundreds of them, the innovation in Formula One is always extreme."
Red Bull team boss Christian Horner was quoted by Germany's Auto Motor und Sport as saying the Mercedes violated the "spirit" of the regulations.
The team said the quotes were incorrect.
Horner, whose Dutch driver Max Verstappen beat Mercedes' seven times champion Lewis Hamilton to the title last year in a controversial final showdown, later told Sky Sports television he had "not paid a great deal of attention" to the Mercedes.
"It's obviously a fairly different concept, but that's for the designer guys and aerodynamicists to get into," added the Briton.
Verstappen said he was not worried that Mercedes, constructors' champions for the past eight years, might have gained an advantage.
"I'm not concerned. We can only focus on ourselves," he told reporters at the Sakhir circuit. "Even if the cars look a bit different, there are so many unknown things that we need to learn about. I'm not worried at all."
Hamilton's new teammate George Russell said it was not an advantage to anyone at the moment.
"As Max said, they (Red Bull) have a few things coming. At the moment Ferrari probably look globally the strongest in every low fuel run," added the Briton.
"They look very, very solid and if you look at their sidepods, they are quite drastically different to what we have at Mercedes and Max has at Red Bull. Time will tell next weekend."
Ferrari's Charles Leclerc topped the timesheets at the lunch break at the Sakhir circuit, with Sergio Perez fourth for Red Bull and Hamilton fifth fastest.
The season starts in Bahrain on March 20.
