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The Ivory Coast international believes that the defending champions need to prove that they have the mental strength necessary to overturn Sir Alex Ferguson's side's six-point lead

Manchester City midfielder Yaya Toure has claimed that Sunday's defeat to Manchester United will only make the defending Premier League champions even more determined to retain their crown.

Robin van Persie scored a dramatic 90th-minute winner for the Red Devils after Toure and Pablo Zabaleta had cancelled out Wayne Rooney's two first-half goals.

And, after suffering that late blow, the Ivory Coast international insists he and his team-mates will need to be "tough" to overhaul United, who are currently top of the table by six points.

"This makes us more determined to win it," he told reporters. "We are tough men, and when things get difficult, that is when you find the tough men.

"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 have to be strong mentally, and strong in every other way as well, but the Premier League is not finished yet.

"We thought we would win it at 2-2, but United have great experience and Wayne Rooney was fantastic for them. That is what United are about - they are a big team and we are proud to play against them.br />
"But this is not finished. We will continue to give them problems in the league, and at the end of the season I hope we will win it again."

Toure admitted that Van Persie's winning goal was difficult for the home side to take but nevertheless acknowledged that the unpredictable nature of the Premier League was one of its biggest assets.

"I am not happy at all," he bemoaned. "In the first 20 minutes to half an hour we dominated the game, but then United got two chances and scored two goals.

"After that we came back again, but it was difficult. We need to improve by working hard, because when you are at the top everybody wants to take you down.

"Their goal was a dagger in the heart - unbelievable. But it was a fantastic game for the fans, and that is why I like the Premier League, it is the toughest league in the world.

"Last year was fantastic, but we have to realise sometimes you will have a bad time. But we have to be strong guys, tough men, and work harder to turn this around."