Updates to change sourcing, dateline; Adds analyst comment in paragraphs 9, 10

Dec 9 (Reuters) - Omnicom Group OMC.N has struck a $13.25 billion all-stock deal to buy rival Interpublic Group IPG.N, creating the world's largest advertising agency as traditional players look to better compete with Big Tech firms amid accelerating use of AI.

The deal, announced on Monday, is expected to attract regulatory scrutiny as it seeks to merge the world's third-largest ad buyer, Omnicom, with the fourth-largest - Interpublic. Both companies are based in New York.

Interpublic shareholders will receive 0.344 Omnicom shares for each share held, or $35.58 based on Omnicom's last close.

Interpublic's shares, down more than 10% year to date, were up nearly 15% in premarket trading. Omnicom fell 4%.

The combined company would have revenue of more than $20 billion, based on 2023 figures. It would compete with the UK's WPP and France's Publicis Groupe SA PUBP.PA, which generated annual revenue of 14.85 billion pounds ($18.97 billion) and 13.10 billion euros ($13.86 billion), respectively.

Interpublic owns brands such as McCann, Weber Shandwick and Mediabrands, and had a market value of around $10.9 billion as of Friday. Omnicom was valued at $20.2 billion and owns the likes of BBDO and TBWA.

Tech giants such as Alphabet-owned Google GOOGL.O and Amazon.com AMZN.O have in recent years attracted marketing dollars away from traditional agencies by offering both advertising tools and marketplaces to buy and sell them.

Soaring use of AI tools that allow businesses to create ads cheaper and faster has also squeezed traditional agencies, forcing them to scramble to develop similar in-house tools to retain clients.

With more tech-driven solutions coming into the market, MoffettNathanson analyst Michael Nathanson said he was concerned the underlying value proposition of an ad agency's offering would remain pressured.

"An integration of this size would be unprecedented and likely challenging. The winners in this kind of transaction could end up being the newco's biggest rivals who would use the deal to try to steal clients and talent," Nathanson said.

Regulatory roadblocks had forced Omincom and Publicis to call off their $35 billion merger, which would have created the world's biggest advertising group, in 2013.

Meanwhile, Publicis' early investments in data and AI technology have helped it weather the changes in the industry better than rivals.

The Wall Street Journal first reported on the Omnicom-Interpublic deal talks.

($1 = 0.7827 pounds)

($1 = 0.9453 euros)

(Reporting by Urvi Dugar, Gursimran Kaur and Disha Mishra in Bengaluru; Editing by Andrea Ricci, Lisa Shumaker and Will Dunham and Sriraj Kalluvila)

((GursimranKaur.Mehar@thomsonreuters.com;))