Manchester City ace Sergio Aguero says they must not underestimate Champions League opponents Schalke.

City's record scorer warned his teammates to ensure that they are at the top of their game.

“Looking at the recent results, we have to be very careful," the Argentine told ESPN.

“We saw strange things happening this week and it helps us to know that we must be very focused on how we handle our game next week at home.

“[There were] so many unexpected results. PSG had a good lead after winning in Manchester and you saw how United advanced.

“The Champions League is very difficult, all teams are very good. In a long competition like the Premier League you can allow yourself some mistakes but in the Champions League you can't. You have to be very focused, be very careful with set pieces and on all the small details."