A top Volkswagen executive did not know about diesel engine emissions test-cheating at the automaker, according to media reports.

Reuters, citing German newspaper Bild am Sonntag, said VW chief executive Matthias Mueller had no knowledge of the carmaker's diesel emissions cheating scandal. The paper cited a report by the law firm investigating the scandal, Jones Day.

Mueller learned of VW's emissions cheating software on 18 September 2015 when US regulators said the German carmaker had faked pollution tests, Bild am Sonntag reported.

Reuters noted Mueller, chief executive of Porsche before the diesel scandal arose, was promoted to VW group chief executive on 25 September and told to turn the automaker around.

A spokesman told the news agency Volkswagen could not comment on an ongoing legal investigation.

Earlier, Reuters reported a US congresswoman had complained, in a letter, a settlement with US regulators would give VW too much authority over how to spend US$2bn on electric vehicle technology.

VW agreed to spend $1.2bn nationally and another in $800m in California on EV technology after being exposed as having falsified diesel vehicles' emissions tests.

On 18 October, a federal judge will consider final approval of $15.3bn in settlements for owners, state and federal regulators or require changes and renegotiation, the news agency said.

A provision "of particular concern" allows VW to make "possible investments in its own proprietary technology and subsidiaries," the congresswoman said in her letter to Environmental Protection Agency (EPA) administrator Lisa McCarthy.