German Chancellor Angela Merkel pressed auto giant Volkswagen Tuesday to assure "full transparency" in a spiralling scandal over pollution test cheating.

"In view of the difficult situation, it is now key to show full transparency (and) clear up the entire matter," she told reporters.

"The (German) transport minister (Alexander Dobrindt) is in close contact with the company, VW, and I hope the facts will come to light as soon as possible."