WASHINGTON — A Federal Aviation Administration plan to consolidate hundreds of outdated facilities isn’t ready two weeks before a deadline set by Congress, potentially delaying a $40 billion program to modernize the nation’s World War II-era air traffic-control system.

Aviation officials told lawmakers Thursday that they haven’t reached agreement on a plan to close, consolidate or realign more than 400 air traffic-control facilities across the country, many of which are more than 50 years old and have fallen into disrepair.

NextGen, a satellite-based air-traffic control system that’s to replace the current radar-based one, is intended to make the skies safer and more efficient. It’s supposed to be complete by 2025, but its implementation depends on the consolidation of air traffic control buildings and facilities, a process that could take two decades. As part of a multiyear reauthorization of the FAA that was signed into law in February, Congress gave the agency 120 days to submit its plan.

Officials from the FAA and the union that represents air traffic controllers will meet Tuesday to discuss the plan, said Paul Rinaldi, the president of the National Air Traffic Controllers Association. That’s nine days before it’s due.

“Nine days is clearly not enough,” Rinaldi said. “But it’s certainly a start.”

David Grizzle, the FAA’s operating chief for air traffic organization, said the plans were complex because they involved changing flight patterns, and the agency wanted to make accurate decisions even if it took more time.

“We can’t make light decisions,” Grizzle said. “If we merely consolidate facilities without restructuring airspace, we may very well set ourselves back.”

Members of the House of Representatives’ Subcommittee on Aviation expressed frustration that the FAA waited until the last minute to finalize the plans. Grizzle said he’d discussed the plans with Rinaldi “maybe a month ago.”

“The FAA knew they were coming in here for this hearing,” said Rep. Jerry Costello, an Illinois Democrat. “When the subcommittee acts, the FAA acts.”

Rep. Peter DeFazio, an Oregon Democrat, wondered whether the agency would have enough time to draft a plan that affects thousands of workers and represents billions of dollars of investment.

“Come on. We’re going to have something comprehensive nine days after you sit down with the people you identify as the principal stakeholders?” he said.

Niel Wright, a spokesman for Republican Rep. Tom Petri of Wisconsin, the aviation panel’s chairman, said Congress wouldn’t give the FAA an extension, and that the Transportation Committee would simply exert pressure on the agency to finish the plan.

“Government agencies need the cooperation of Congress, so they generally try to cooperate in return,” Wright said.

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Rep. John Duncan, a Tennessee Republican, noted that the FAA had completed only two of the seven terminal facility realignments it identified two years ago. A Texas consolidation that was supposed to take place this year has been delayed until next year, and others in Michigan, Ohio and Illinois are on hold. A plan to move a West Palm Beach, Fla., facility to Miami was canceled, and a new facility will be built in West Palm Beach instead.

“It looks to me and almost everyone else that little progress has been made,” Duncan said.

Grizzle said combining two existing facilities into one might be more expensive than keeping them separate, depending on the location. It can take several years to transition from an old facility to a new one, he said.

“In many instances, it’s the right thing to do, and in others, it’s not,” Grizzle said.

The FAA is planning to start the consolidation process in the notoriously congested airspace of the New York region, a project that will place high-altitude and low-altitude controllers under one roof.

The FAA estimates that it will cost $2.3 billion to construct its first four integrated facilities but that it has only $700 million set aside for them.

With federal funds tight, lawmakers pressed the agency for accurate estimates.

“I would hope that the FAA, working with the stakeholders, comes up with a plan that measures the true cost,” Costello said.