With completion scheduled for early 2018, the 17-story 121 Seaport office tower on the South Boston waterfront is still looking for its first tenant.

Developer Skanska USA is open to a variety of leasing options, said Charley Leatherbee, executive vice president and leader of its development operations in Boston.

“A full building tenant is always a good thing to have, but the great thing about this building is we have tremendous flexibility and can offer any combination. At the moment we are pursuing larger-sized tenants,” Leatherbee said following at a topping-off ceremony Friday to mark the completion of the building’s steel frame.

Demand for Seaport office space – from downtown, suburban and out-of-state companies – shows little sign of diminishing following commitments by General Electric and Reebok last year to move their global headquarters to the neighborhood.

Amazon will expand from Cambridge into Boston this year with plans to occupy WeWork’s space at 31 St. James Ave. in Back Bay. Leatherbee said 121 Seaport could be an option if the e-commerce giant pursues a long-term office deal in the city.

“If you believe that Amazon’s going to add 100,000 employees over the next few years and they want to put that flag in Boston, this would be a good building for them to be in,” Leatherbee said.

Designed by Boston-based CBT Architects, the 400,000-square-foot 121 Seaport contains 15 floors of office space atop a two-story retail podium.

Promoted as Boston’s first elliptical tower, the design squeezed efficiencies out of its chilled beam heating and cooling system, said David Nagahiro, a principal at CBT. That eliminated the need for a large fan room in the core space on each floor, enabling nearly 86 percent of each floor to be leasable space.

“This is the most efficient floor plate in the city,” Nagahiro said.

Skanska’s decision to build 121 Seaport on speculation followed its success attracting PwC’s regional headquarters as anchor tenant for the neighboring 101 Seaport, a 440,000-square-foot office tower that it sold to German real estate fund Union Investment Real estate GmbH for $452 million last year.

Newmark Grubb Knight Frank is representing Skanska in the 121 Seaport leasing negotiations.