Manchester City manager Manuel Pellegrini said he could not explain his team's disintegration after seeing them blown away 4-2 by Manchester United in the Premier League on Sunday.

The defeat, City's sixth in eight games in all competitions, left last season's champions 12 points behind leaders Chelsea having played a game more and four points adrift of third-place United in the contest for an automatic Champions League qualifying berth.

It is likely to increase speculation about Pellegrini's position at the Etihad Stadium, a year after he led City to a league and League Cup double, and he was at a loss to explain his side's shortcomings.

"I don't understand why a team only plays so well in the first 20 minutes," the Chilean told Sky Sports at Old Trafford.

"We dominated, created chances, we had the ball. At 1-1 United started growing and their third goal gave them the confidence they did not have before.

"It is not just one problem. The same team played well for 20 minutes. Maybe it's a lack of class at this moment. I don't know."

Asked if he was worried about his future at City, he replied: "I'm not talking about my position."

City were bidding to record a fifth consecutive Premier League victory over their derby rivals and they made an ideal start when Sergio Aguero tapped in David Silva's eighth-minute cross in front of the Stretford End.

But goals from Ashley Young and Marouane Fellaini saw United go in 2-1 up at half-time and further efforts from Juan Mata and Chris Smalling swelled the hosts' lead before Aguero hit back at the death with his 100th City goal.

United have now won six league games in succession -- including convincing wins over Tottenham Hotspur, Liverpool and now City -- and will approach Chelsea's visit next weekend brimming with new-found confidence.

"It's three points. We are four points ahead of fourth in the table and I am very happy for the fans," cheered United manager Louis van Gaal.

"I have seen what it means for them. They always support us when the results were not good. It's the best atmosphere I have seen here. A fantastic result."

United finished the game with 10 men after Michael Carrick was forced off by injury, Van Gaal having already made his three permitted substitutions, and the Dutchman admitted he had made a miscalculation.

"I must bring Michael Carrick off the pitch with an injury, which was a bit stupid from me as some players didn't know we only had 10 men," he said. "That was not so smart!"