German car parts maker Bosch helped develop and conceal the so-called defeat devices used by Volkswagen to cheat on emissions tests, lawyers acting for US plaintiffs said in newly released court documents.

Both firms are being targeted in a mass lawsuit by US car owners affected by the "dieselgate" scandal, which erupted last year after VW admitted to installing manipulating software in 11 million vehicles to make them appear less polluting during lab tests.

Bosch "played a critical role" in the scheme to evade US emissions requirements, lawyers wrote in a San Francisco court filing seen by AFP Wednesday.

The document says Bosch wrote to VW in 2008 and demanded that it indemnify Bosch "for anticipated liability arising from the use of the Bosch-created 'defeat device'", which it knew to be illegal.

Volkswagen refused the request but Bosch nevertheless continued to develop the cheating software, it said.

Bosch also "concealed the defeat device in communications with US regulators once questions were raised about the emission control system".

"Bosch was a knowing and active participant in the decade-long illegal enterprise to defraud US consumers," the lawyers claimed.

While Bosch had been named in the class action lawsuit before, the updated court filing lays out the first detailed allegations against the Stuttgart-based company.

A Bosch spokesman said the firm was still working on a formal response to the accusations.

"Bosch takes the allegations of manipulation of diesel software very seriously," he told AFP.

Bosch is also facing legal woes at home, with prosecutors in Stuttgart also investigating its possible role in the scandal.

Bosch previously said it was carrying out its own internal inquiry into the controversy and that it had set aside funds to cover the fallout from any potential legal claims.

Volkswagen has already agreed to a roughly $15-billion settlement to compensate owners of some 480,000 cars in the United States.

The vast majority of the "dieselgate" cars were sold in Europe however, where Volkswagen so far has only offered a broad recall to fix the vehicles' software.