The U.S. Senate on Tuesday failed to end a partial shutdown of the Federal Aviation Administration (FAA), leaving around 4,000 agency employees furloughed, possibly until September.

The partial shutdown was due to an impasse over temporary funding for the FAA. It began on July 23 after the Senate failed to approve an extension of taxes that help fund the agency. The measure was passed by the House.

Although partially shutdown, air traffic controllers, who are paid with separate funds, will continue to work. Transportation Secretary Ray LaHood said, "No safety issues will be compromised."

"Flying is safe. Air traffic controllers are guiding airplanes. Safety inspectors are on duty and are doing their job. No one needs to worry about safety," he told reporters on a conference call.

The stalemate centers on disagreements between Republicans and Democrats over a program that subsidizes commercial air service to rural airports. But reports indicated the real fight is over federal rules on labor elections in the airline industry.

The House is already in recess, and the Senate is following suit late Tuesday or Wednesday. Lawmakers will not return to legislative business until September.