The decision backs Google's stance that it is focused on classified internet advertising while DoubleClick is in the different market of display internet advertising.
The EU has already extended the deadline for its decision on the deal from October 26 to November 13, but Michael Lee, the IAA's executive director, has written to European competition commissioner Neelie Kroes to suggest a patient review.
Lee said: "Unquestionably this proposed transaction, like other recently announced internet-related deals is complex, with many points of view concerning competition in the market, as well as potential data protection and privacy concerns.
"The European Commission need not be hurried into a decision. There is much at stake for advertisers and consumers alike."
The intervention of the IAA, which encompasses 76 countries, comes after the World Federation of Advertisers last week urged the EU to "closely examine" the deal. The WFA represents advertisers, which together account for around 90% of global marketing spend.
The EU will this week receive back the questionnaires it sent out to customers of Google and DoubleClick to find out their views on how the merger would stifle competition, according to Dow Jones Newswires.
The US competition authority, the Federal Trade Commission, is also scrutinising the deal.
US regulators have already approved other similar deals including Microsoft's $6bn acquisition of aQuantive and Yahoo!'s $300m acquisition of Blue Lithium.