Yaya Toure has vowed Manchester City will 'continue to give United problems in the league' despite losing Sunday's Manchester derby.
City looked to have rescued a point when they fought back from 2-0 down to draw level with less than five minutes of normal time remaining, but Robin Van Persie struck in injury time to hand United victory.
The win saw United increase their lead at the top of the Premier League to six points over the neighbours but Toure insists the title race is far from over and he has urged his team-mates to show the spirit that saw them snatch the title in May.
"The title race is not finished - anything but. We will continue to give United problems in the league and at the end of the season I hope we will win it again," Toure told City's official website.
"We thought we would win the game when we made it 2-2, but United have great experience and Wayne Rooney was fantastic for them.
"It was a dagger in the heart to concede a third so late in the game, but it was a fantastic game for the fans, and that is why I like the Premier League so much. It is the toughest league in the world.
"Disappointing though this defeat is, it only makes us more determined to win the title. We have to be strong mentally and strong in every other way as well, but our championship challenge is not finished."
City overcame an eight point deficit to win the Premier League title on goal difference but they have struggled at times this season and will be looking to bounce back immediately.
With over half of the season remaining Toure knows there is plenty of time to recover and has warned it will prove difficult to retain the title.
"We have to forget what we have done in the past. This year will be tough, but we have a fantastic squad and we have the confidence to keep going until the final day. We will never give up," he added.
"We need to improve by working even harder, because when you are at the top everybody wants to take you down.
"Last year was fantastic, but we have to realise that sometimes you go through tough periods - we can turn this around. When the chips are down, that's when the tough really get going and we will show why we are champions."