The agencies said in the release that more than half of United States consumers now started online product searches on Amazon, compared with 28 percent on search engines and 16 percent on retailer websites.

Martin Sorrell, WPP’s chief executive, said on an earnings call this year that “Amazon’s penetration in most areas is frightening to some.” He added that the company was his response when people asked him, “What worries you when you get up at night and when you wake up in the morning?”

In an interview in Cannes, France, last month, Mr. Sorrell said his firm wanted to do more with clients and Amazon, but noted there were major questions around how brands might gain access its customer data and compete on voice search.

“What happens if I say to Alexa, ‘I like Cheerios,’ and Alexa says, ‘I’ve got Kellogg’s Corn Flakes, which are 10 percent off’?” he said.

Amazon Media Group, the company’s growing advertising division, has been looking to assuage such concerns while touting new ways marketers can reach people on Amazon. Seth Dallaire, its head of global ad sales and marketing, has been urging agencies to view product pages and images as “brand marketing vehicles,” noting that if they are not well maintained it could undermine all the work companies did to get people there in the first place.

“We can see the entire customer-decision journey, and that’s what’s unique,” Mr. Dallaire said. “We can help a brand if they’re selling their products on Amazon understand when a customer is exposed to an ad and, when they clicked on an ad, if they bought something, and then we can help them tailor their marketing messages and their creative to each different step.”

That’s no small pitch given the guesswork that goes into advertising. Amazon has long sold lucrative sponsored product listings and other ads tied to search terms on its site, like Google. It also offers automatically placed ads on external sites using its own technology and data, and marketing on Amazon packages and devices like Kindles.