WASHINGTON, Sept 14 (Reuters) - The Federal Aviation Administration needs more funding ‌from Congress to complete phase ‌one of a $10.6 billion air traffic control ​reform plan and has underestimated the total costs of operating the new system, a government report said Monday.

The ‌Government Accountability Office ⁠said the FAA had not fully assessed the costs ⁠of operating the new system or set a detailed schedule for ​completing reforms.

"In ​essence, the FAA ​is buying delivery ‌speed at the risk of cost uncertainty and potential disruptions," the GAO report said, adding "these risks increase the likelihood of FAA repeating its ‌history of cost overruns, ​delays, and unmet performance ​targets."

Congress ​previously approved $9 billion for ATC ‌reforms in the first ​phase set ​to cost $10.6 billion. The FAA is using $1 billion from its current ​year ‌budget and needs another $574 million, the ​GAO said.

(Reporting by David ShepardsonEditing ​by Nick Zieminski)