LONDONArsene Wenger admits Arsenal's defensive frailties are beginning to damage his team's push for a place in the top four of the Premier League.Some sloppy defending, most notably by the Gunners' German centre-back Per Mertesacker, contributed to Arsenal's surprise 3-2 defeat at Swansea on Sunday and Wenger refused to protect his back four.Successive league defeats have left Arsenal four points behind fourth placed Chelsea and 10 behind third placed Tottenham, putting them in real danger of failing to qualify for the Champions League.Wenger acknowledged the defence needs to improve quickly and said: "I felt that there were some odd turning points, like when we went to 2-2 we straightaway made a defensive mistake."It was similar for me to the Fulham match, a lack of appreciation of the ball. Our defensive performance was not good enough and that is why we lost the game."When we came back to 2-2 I thought we had a good chance to win the game because we looked dangerous.Wenger also criticised referee Michael Oliver for awarding a penalty to Swansea which he described as "complete imagination".Robin van Persie had given the Gunners a fifth-minute lead only for Oliver to point to the spot just 11 minutes later when Nathan Dyer went to ground under pressure from Aaron Ramsey, which Wenger branded a "good dive".Ends