Amazon is on the hunt for as much as 1 million square feet of office space in Boston, adding a new level of intrigue to the retail giant’s plans as it whittles down the list of the cities competing for its second headquarters.

The Seattle company is negotiating with a Seaport developer to lease an entire office building, and possibly two, to continue its expansion in the city, according to real estate industry executives with knowledge of the talks.

Amazon employs more than 1,000 people in Boston and Cambridge — mostly software engineers and developers — with offices open or under construction in Kendall Square, the Back Bay, and Fort Point. It expects to nearly double that number over the next few years, and could grow even larger here with more space in the Seaport.

Amazon’s Seaport search reportedly began months before the company announced in September that it wants to build a second headquarters — envisioned as a vast campus big enough for 50,000 workers — and before Boston emerged as a top contender for that project. And it is being conducted separately from the headquarters competition, sources say.

Amazon has zeroed in on Seaport Square, a planned complex of office buildings and housing along and near Seaport Boulevard for which Chestnut Hill-based WS Development ­recently won city permits. Amazon and WS are in talks about 500,000 square feet of office space, with an option to later lease a second building of about equal size.

A decision is expected in the next few months, sources say, meaning it will be made around the same time as Amazon winnows the 238 headquarters proposals it received to a handful of finalists.

It aims to pick a site for its so-called HQ2 by year’s end, and move into a first phase of roughly 500,000 square feet of office space by the end of 2019.

At this point, it’s impossible to say how the Seaport talks, which were reported earlier this week by the Boston Business Journal, and the headquarters search are related, said Brendan Carroll, director of intelligence at the real estate firm Perry Brokerage Associates.

“We don’t really know what the form and substance of HQ2 is going to be,’’ Carroll said.

“It would be an incredible coincidence if they were in the market for the exact space as they want in their first phase [of the headquarters project]. But who knows?’’

For now, Amazon isn’t dropping any clues about its strategy. A spokeswoman for the company said Wednesday that she had no information on any growth plans in the Seaport.

Officials at WS and Boston City Hall declined to comment.

Second headquarters or not, the company is gobbling up office space in Boston and elsewhere as its workforce expands rapidly.

Five years after opening a small office in Kendall Square, Amazon today fills much of a building on Broadway in Cambridge, leases a large chunk of a WeWork co-working space in the Back Bay, and is renovating 150,000 square feet of office space in Fort Point that is set to open this spring.

Amazon probably wants to keep growing here regardless of its headquarters decision, and could be seeking room do to just that, according to Carroll and other experts in the real estate industry.

A year ago, the company said that it planned to create 100,000 jobs in the United States by mid-2018.

Boston is among the largest of a dozen “tech hubs’’ — beyond Amazon’s Seattle home — where Amazon is hiring at a rapid clip as it expands its video streaming and Web server businesses and deepens its expertise in artificial intelligence, machine learning, and other tech-oriented fields.

“They’re like an octopus expanding with all these different tentacles,’’ said Michael A. Cusumano, a management professor at the Massachusetts Institute of Technology who has studied the company. “They don’t seem to place a lot of limits on what they do.’’

In making a pitch for Amazon to bring its second headquarters to Boston, the Walsh administration leaned hard on the region’s talented workforce.

As for real estate, it focused its proposal on Suffolk Downs, noting that the shuttered East Boston racetrack is the only site in the city big enough to accommodate the 8-million-square-foot campus Amazon says it wants.

Suffolk Downs’ developers are pushing to start construction this year on a pair of office buildings, about 500,000 square feet combined, to have a first phase ready for Amazon in 2019.

The city’s Amazon pitch listed the Seaport as a secondary option for Amazon’s campus. Backers of that neighborhood note that it already is home to a slew of big-name technology and professional services companies, and that it sits closer to the heart of the city.

On its own, WS’s Seaport Square project isn’t big enough for the 8 million square feet Amazon says it needs for HQ2. The developer won approval last fall to build 2.8 million square feet of office space on four blocks, from Northern Avenue to Summer Street, with construction on the first building likely to begin at 88 Seaport Blvd., which is next to District Hall.

But other Seaport developers have also floated Amazon plans, including a partnership of the Millennium Partners subsidiary Cargo Ventures, Related Beal, and Skanska USA.

Combined, they control several million square feet of developable space in the nearby Raymond L. Flynn Marine Industrial Park.

Carroll said that he could envision a headquarters campus that would begin at Seaport Square and expand east toward the waterfront industrial park.

“I don’t think there’s any guarantee that this isn’t Amazon’s HQ2,’’ Carroll said. “It’s very, very interesting.’’

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.