US President Donald Trump stepped up a series of attacks on automakers for not backing his administration's plan to roll back Obama-era fuel efficiency rules.

He singled out Ford in particular for backing a deal with California for stricter fuel economy standards.

Ford is one of four automakers, along with Honda, BMW and Volkswagen, that reached a voluntary agreement with California on fuel efficiency rules, defying Trump and his administration's effort to strip the state of the right to fight climate change by setting its own standards.

The rules under the California plan are looser than the Obama-era regulations but stricter than what the Trump has proposed.

Trump said company founder Henry Ford would be "very disappointed if he saw his modern-day descendants wanting to build a much more expensive car, that is far less safe and doesn't work as well, because execs don’t want to fight California regulators."

Ford said in a statement that it is focused on acting to protect the environment while also protecting the affordability of vehicles. 

"This agreement with California provides regulatory stability while reducing CO2 more than complying with two different standards," it said.

There is no evidence that existing fuel economy rules would degrade vehicle performance. And environmentalists and many states challenge Trump's assertion that his administration's proposed rule would boost vehicle safety or dramatically reduce the price of vehicles, and argue that consumers will save more in reduced fuel costs under the Obama rules.

California Attorney General Xavier Becerra responded to Trump's attacks on automakers saying it would result in an additional 540 million metric tons of greenhouse gases and other harms. 

"This doesn't look like a better alternative to us," he said.

The White House has urged other automakers not to back the California agreement, while Democrats have been calling and writing automakers urging them to sign on with California.

The Environmental Protection Agency ridiculed the voluntary framework, which it said "so far has been nothing morethan a press release."

"My proposal to the politically correct Automobile Companies would lower the average price of a car to consumers by more than $3000, while at the same time making the cars substantially safer. Engines would run smoother. Very little impact on the environment! Foolish executives!" Trump tweeted earlier.

Gloria Bergquist, a spokeswoman for the Alliance ofAutomobile Manufacturers, representing General Motors, Toyota, Ford, Volkswagen and others, said the companies "look forward to seeing a final rule soon."

GM has not backed the voluntary agreement, arguing that it does not properly credit the company's electric vehicles.

Even so, Trump tweeted that the founders of Ford and GM "are 'rolling over' at the weakness of current car company executives" over the fuel rules, adding: "Crazy!"

GM did not immediately comment.