Liverpool boss Jurgen Klopp has said you cannot count out Manchester City in this year's Premier League title race, with Pep Guardiola's side slipping to six points behind leaders Arsenal.

City suffered a 1-0 defeat away to Aston Villa on Wednesday -- their fourth straight league game without a win -- that leaves them in fourth place.

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However Klopp, whose Liverpool sides have been pipped by City to the Premier League title on more than one occasion, said dismissing the treble winners as challengers would be a huge mistake.

"If somebody would write City off that would be the biggest joke in the history of football," Klopp said after Liverpool's 2-0 win over Sheffield United on Wednesday.

"Arsenal are fighting and had a similar experience [to what] we had," Klopp added, referencing Arsenal's 97th-minute winner to beat Luton Town 4-3 on Tuesday. "It feels really good but anyway you have to start the next game from 0-0.

"Aston Villa is incredible. [Manchester] United is there, don't play yet maybe exactly the football whoever wants -- the media or whatever -- but they have points like crazy."

Guardiola admitted his side are "struggling" to rediscover their title-winning form as they seek an unprecedented fourth-straight Premier League trophy.

"We know what's our level and as a manager we have to find a way to win games. We're struggling a bit. I'm here to help them," Guardiola said.