Millions of dollars are being wasted due to poor management of government-owned vehicles and aircraft, two new reports by the Government Accountability Office have found.

The report on the federal vehicle fleet was prompted by long-running concerns that the fleet is bloated beyond the agencies' needs.

According to the GAO, agencies spend about $1.7 billion a year to run a fleet of about 387,000 vehicles. The report found that many agencies could not justify the number of vehicles owned against what they require to carry out their work.

In one case, the Department of Interior's inspector general told the GAO that a significant number of the 36,000 vehicles it owned were underutilized and estimated annual savings of $34 million if these vehicles were trimmed from the fleet.

In another highlighted case, a car had been parked behind a Department of Veterans Affairs laundry since it was purchased four years earlier. The report said the vehicle had never been used and its keys were missing.

"It is unacceptable for federal agencies to buy vehicles they don't need and don't use," Sen. Susan Collins (R-Maine), chairwoman of the Senate Governmental Affairs Committee, said in a statement. "Financial management continues to be a weak point for the federal government. We must demand that federal agencies follow responsible fiscal practices."

In its report on federal aircraft operations, the GAO said it was unable to work out the composition and costs of the fleet because the government aircraft database, as maintained by the General Services Administration, was "unreliable because it was incomplete and inaccurate." The report found the database had understated the cost of the federal air fleet by at least $568 million over the last three years.