Speculation about where Boeing will build its new 777X long-range jet is beginning to ramp up.

Reuters new service reported Monday that the aerospace giant is in talks with the International Association of Machinists to assemble the latest version of the 777 passenger plane and construct the advanced composite wings in the Seattle area, the longtime home of its commercial aircraft business.

The union confirmed that later in the day to the Wall Street Journal. Tom Buffenbarger, international president of the machinists, told the Journal that the talks are “at a critical stage” and that more details could be known in “the next 24 to 48 hours.” The newspaper reported last week that Boeing was considering its North CHarleston campus as the manufacturing site for the 777X.

Reuters, citing an unnamed industry source, reported the proposed deal with the machinists union would put final assembly in Washington state. Talks are “in the final stages, but are not done yet,” the person told Reuters, adding the negotiations, which were initiated quietly between Boeing and the machinists union around a week ago, were “intense.”

Reuters reported that Boeing is expected to hold similar negotiations with other potential locations.

A Forbes blog posting added another layer of intrigue. It said Boeing is eyeing a facility for 777X production in Long Beach, Calif., where the C-17 program will be mothballed in 2015.

The Forbes report, which did not cite a source, said Boeing is looking to give a new role to the 5,000 workers there once production of the military cargo plane is idled.

“The feasibility of using the Long Beach workforce arises from the fact that it already has deep experience in building large airframes that incorporate lightweight, high-strength composite materials, and the site will become available just as engineering and production of the 777X begins ramping up in the second half of the decade,” defense analyst Loren Thompson wrote in the Forbes report.

Boeing South Carolina spokeswoman Candy Eslinger said the company would not comment on all the speculation.

The 777X is expected to begin production in 2017 or 2018 and take its first flight by 2020. The plane is expected to hold more passengers, fly farther and use 20 percent less fuel than the existing 777.

Last week, Boeing disclosed that engineers in Long Beach as well as North Charleston and three other sites across the country and one in Moscow would handle most of the detailed design work for the new airplane.

The company is buying 267 acres at Charleston International Airport for future expansion. Boeing has not said what it intends to do with the property. The sale is expected to close later this month.

The state hasn’t been in talks with Boeing for an incentive deal strictly to land the 777X, Charleston County Aviation Authority airports director Paul Campbell said Monday. Campbell, who also is a state senator, was involved in other incentive packages for Boeing in South Carolina.

The state offered Boeing a $450 million incentive deal in 2009 to lure Boeing’s second Dreamliner plant to North Charleston. The other line is in Washington state.

A second incentive package was announced in April. Under that deal, Boeing said it would create 2,000 more jobs and invest another $1 billion at its North Charleston site by 2020 through a $120 million package of state assistance.

Reach Warren L. Wise at 937-5524 or twitter.com/warrenlancewise.