German prosecutors are pressing criminal charges against three current and former Volkswagen executives in connection with the diesel emissions scandal.

The current CEO and chairman, plus a former CEO, have been charged with stock market manipulation. The trio are accused of intentionally delaying telling investors about the car-maker's cheating of U.S diesel emissions tests.

Prosecutors in the city of Braunschweig said on Tuesday they aimed to prove VW chairman Hans Dieter Poetsch, CEO Herbert Diess and Diess's predecessor Martin Winterkorn guilty of stock market manipulation.

Four years after the German company admitted using illegal software to cheat U.S. diesel engine tests, the charges show it is still struggling to move on from a scandal which has cost it more than $30 billion in vehicle refits, fines and provisions.

Court proceedings are underway over that admission from September 2015. The indictment from the prosecutors in Braunschweig - in Volkswagen's home region of Lower Saxony - is part of a separate legal push to try managers over allegations they delayed disclosing the scandal to investors.

Volkswagen has said it is confident the allegations will prove groundless. Lawyers have said Diess, who did not join VW until July 2015, will continue as CEO. Winterkorn stepped down after the scandal, with Diess replacing him. Lawyers for Winterkorn and Poetsch have said their clients are blameless.