Aguero said Monday on his official website that he wants to take "a new step" in his career after five seasons in the Spanish capital.

"I said some time ago that when I wanted to leave I would make it known publicly. Now that moment has come," Aguero wrote. "It's difficult to leave Atletico ... the reasons for doing so are difficult to explain." Real Madrid and Chelsea have long been linked with a move for the 23-year-old Aguero, with Atletico rejecting offers from both clubs in January. Aguero made no mention of where he would like to play next.

Aguero said he wanted to continue his development as a player after helping Atletico to a seventh-place league finish this season.

Atletico needs to play a qualifying round just to reach the Europa League, so the prospect of playing Champions League football may have pushed Aguero to seek an early exit from a contract that still has three years to run.

"It's not for economic reasons but strictly sporting ones. This decision has been brewing for some time," said Aguero, who added that he signed a contract renewal so Atletico would not miss out on a payout if he left.

The former Independiente striker, who joined Atletico in 2006 for ¤22 million (then $28 million), scored 27 of his 101 goals this season, including 20 in the league. He helped Atletico to Europa League and European Supercup triumphs last season.

Alongside goalkeeper David De Gea, Aguero forms the young and talented core of the club.