Automakers called the European Union's 2030 target for a 30 percent reduction in CO2 emissions ambitious and aggressive.
The reduction level goal is "overly challenging," and goes beyond the EU's wider climate and energy framework, ACEA, the European auto industry's lobby group said in a statement.
Separately, Germany's VDA lobby group said it views the target as hard to meet. It said the goal risked putting European automakers at a disadvantage in global markets.
"From today's perspective, it is more than questionable whether the proposed CO2 targets are achievable. That will depend largely on how fast customers will accept alternative engines in the coming years and how fast public infrastructure is built," VDA said in a statement.
The EU proposal calls for the reduction of 30 percent of average CO2 emissions by 2030 from car makers' fleets compared with 2021 levels. It also sets an interim goal of a 15 percent reduction by 2025 to help ensure automakers kick start investment early.
The EU's current caps on CO2 from cars are 130g/km set for 2015 and 95g/km fixed for 2021. The existing limits are averages for the EU fleet as a whole, with individual manufacturers having specific targets backed by financial penalties.
BMW CEO Harald Krueger said the targets will be tough to meet. "What is now coming out of Brussels is very very ambitious," Krueger said on Wednesday at the annual conference of Automobilwoche, a sister publication of Automotive News Europe.
The current limit of 95g/km 2021 goal cannot be reached without an adequate share of diesel-engine vehicles, Krueger said, adding this will probably also apply to the newly-announced 2025 interim target.
Volkswagen Group division Skoda may be forced to further expand its offerings of fuel-efficient vehicles because automakers missing their targets will face heavy fines in the millions of euros.
"This will mean that we will once again have to sharpen our product strategies," Skoda CEO Bernhard Maier said at Automobilwoche's conference.
Reuters and Bloomberg contributed to this report