NORTH CHARLESTON — Alenia Aeronautica of Rome will decide by June whether to build a new Army cargo jet at a site near Charleston’s airport or elsewhere, assuming the company wins the military contract.

The plant would employ at least 250 people, but only if the Army picks Alenia and orders enough jets to justify a new plant outside Italy, company officials said Monday.

Alenia and Vought Aircraft Industries of Dallas visited the site during a groundbreaking ceremony for a $560 million, 645-job plant that will assemble the aft fuselage of Boeing’s new passenger jet. Previously called the 7E7, Boeing has designated the midsize jet as the 787 Dreamliner.

The companies are negotiating with Boeing to bring additional painting and subassembly work that might add 25 to 70 jobs to the crew that will start production in early 2006, said Tom Risley, chief executive of Dallas-based Vought. Hiring will begin this summer for jobs that are expected to pay $40,000 to $50,000 per year, averages that include hourly and salaried workers.

For now, significant new jobs are riding on another jet: the C-27J Spartan cargo plane built by Alenia and Lockheed Martin. The Army will take proposals this spring from Alenia-Lockheed and rival European Aeronautic Defense and Space Co., which owns Airbus and is offering the C-295. Those proposals must include a plan stating where the planes will be built, said Giuseppe Giordo, the Alenia executive in charge of its North American operations.

After the groundbreaking, Gov. Mark Sanford cited the C-27J cargo jet as a chief example of future jobs and investments stemming from the initial Vought-Alenia venture.

Sanford said the cargo jet has been well received by (Army) special forces and other branches of the military.

Alenia was building 16 of the jets at a subassembly plant near Naples and a final assembly plant near Turin in December. The Army is considering the C-27J as a replacement for the aging C-23 Sherpa used by National Guard units in 20 states, according to the Interavia Business & Technology trade journal.

Pentagon officials said last fall they were looking for a cargo jet that can take off in a short distance, allowing the jet to take over some jobs handled by helicopters in Iraq and Afghanistan.

When the Vought-Alenia plant was announced Dec. 1, Giordo said Charleston would be the lead U.S. contender for a U.S. C-27J assembly plant and that the company would know by 2006 whether it had won the Army contract, worth as much as $5 billion. On Monday, he said the site choice would precede the contract, which might be awarded as early as this fall.

Giordo and other Alenia officials wouldn’t say how many planes the Army would need to order to justify a U.S. plant, except to say that if the Army ordered, say, two planes, they would be built in Italy. The officials said they were encouraged by Pentagon comments last fall that the Army would like to order 33 jets to retire the C-23 by 2011.

The 380-acre site by the Charleston airport offers room for a final assembly plant for the C-27J, Giordo said. He would not identify what other U.S. sites are being considered.

Meanwhile, Alenia and Vought will be busy making fuselages for the Boeing 787.

South Carolina will provide $116 million in incentives. The state was prepared to offer $131 million had Vought and Alenia won from Boeing additional work involving the nose of the 787, work that would have added 300 more jobs in the startup.

Risley said that arrangement fell through about two weeks ago but said the site has a better than 50 percent chance of winning the 25 to 70 other subassembly and painting jobs.