Transportation Secretary Sean Duffy said Tuesday the federal government is making early progress on a long-delayed overhaul of the nation’s air traffic control system, but warned the effort could stall without additional funding from Congress.

Speaking at a Department of Transportation event outlining roughly the first year of work on the $12.5 billion initiative, Duffy said the administration has moved quickly to replace outdated infrastructure and begin building what he described as a “brand new air traffic control system.”