Atletico Madrid have tabled a £10million offer for fellow Argentine Eduardo Salvio, who is seen as Aguero's successor both for club and country.
The Spanish side want to tie up a deal quickly with Salvio's club Lanus and have started to rush through the talks as they know Aguero may be on his way to the Premier League.
Chelsea have been after Aguero for weeks but have run into a major snag over his personal terms. While, super-rich City are flexing their muscles, with their Arab management keen to restore some face after the Mark Hughes sacking debacle.
And neighbours United - who have money to spend on the right player - are watching developments and waiting to see what the price is before entering the bidding.
Sir Alex Ferguson wants an extra attacker and has been on Aguero's trail for some time. There are doubts at United about whether he is physically strong enough for English football, but they still remain interested.