Questions raised over sudden resignation of US Volkswagen chief executive Michael Horn

Michael Horn, president and chief executive officer of US Volkswagen has resigned "effective immediately" although the German carmaker did not explain the reason for his departure.

Horn said the decision was to allow him "to pursue other opportunities" but it comes about six months after VW got engulfed in the diesel emissions scandal and amid a struggle to agree a settlement with the US government.

Horn, who has been the face of Volkswagen in the US since the scandal broke, had been appointed in the lead role in January 2014. While the company did not associate the scandal with his departure, it said Horn was leaving the company "through mutual agreement".

He is expected to be replaced by Hinrich Woebcken, who was recently given the lead role of heading the carmaker's North American business.

Herbert Diess, head of the core Volkswagen brand, personally thanked Horn for his work at the company. "During his time in the US, Michael Horn built up a strong relationship with our national dealer body and showed exemplary leadership during difficult times for the brand," Diess said.

Michelle Krebs, an analyst at Autotrader.com said the move was Volkswagen's attempt at starting with a clean state. "But still what is lacking is a clear plan for helping owners of these vehicles and a plan forward for the brand," Krebs said.

Ever since the emissions scandal came to light in September 2015, Horn had often shown frustration at the group's management in Germany. One instance of his irritation was during an October hearing when he was asked if it was possible the scandal was because of a small group of engineers rather than a corporate decision, to which Horn responded: "I agree it's very hard to believe. Personally, I struggle too."

In another instance, he had admitted that Volkswagen had "totally screwed up" over the vehicle emissions scandal. Speaking at the Passat launch in September 2015, Horn had said: "Our company was dishonest. We have totally screwed up. We must fix the cars to prevent this from ever happening again and we have to make this right.

"This kind of behaviour is totally inconsistent with our qualities. We are committed to do what must be done and to begin to restore your trust. We will pay what we have to pay."

The Wolfsburg-headquartered company has admitted to having fitted its Audi and Porsche diesel vehicles with "defeat devices" to cheat emission tests. These were then marketed as being more environmentally friendly. While about 600,000 of their cars were affected by this scandal, Volkswagen said the worldwide count was about 11m vehicles.