WASHINGTON (AP) — Congressional Republicans are warning Transportation Secretary Ray LaHood he must cut subsidies for air service to 10 rural communities unless he can show it would be too difficult for residents to reach a larger airport.

White House assurances that the cuts would be waived cleared the way for Senate passage of legislation this month, ending a two-week partial shutdown of the Federal Aviation Administration.

But in a letter released Friday by the House Transportation and Infrastructure Committee, Republicans requested that LaHood provide a "full, written justification" of any waivers of subsidy cuts.

The letter was signed by Rep. John Mica, R-Fla., chairman of the committee, and GOP Sens. Tom Coburn of Oklahoma, Jim DeMint of South Carolina, Rand Paul of Kentucky and Mike Lee of Utah.

A dispute between the House and Senate, in part over the subsidies, forced a partial shutdown of the FAA beginning on July 23 that caused the furlough of nearly 4,000 federal workers, halted work on more than 200 airport construction projects and cost the government about $400 million in uncollected airline ticket taxes.

House Republicans sparked the dispute by adding a provision cutting $16.5 million in air service subsidies to a bill temporarily extending the FAA's operating authority through mid-September. Senate Democrats refused to accept the House bill, saying Republicans were breaking with precedent by including policy changes that hadn't been agreed on to an extension bill.

The bill would eliminate subsidies for 10 communities that are less than 90 miles from a hub airport: Morgantown, W.Va.; Athens, Ga.; Jamestown, N.Y.; Bradford, Pa.; Hagerstown, Md.; Jonesboro, Ark.; Johnstown, Pa.; Franklin/Oil City, Pa.; Lancaster, Pa., and Jackson, Tenn. However, the bill also gives the transportation secretary the authority to waive the cuts if the geographic characteristics of a community would make it too difficult for residents to reach a larger airport.

The bill also eliminates subsidies for air service to Alamogordo, N.M.; Ely, Nev., and Glendive, Mont., because the subsidies average more than $1,000 per passenger.

Congressional committees have identified the 13 communities as the cities that would be affected by the law. But a Transportation Department spokeswoman said the department is still working on interpreting the law and which communities might be impacted.

The Essential Air Service program was created to ensure service on less profitable routes to remote communities when airlines were deregulated in 1978. But the program's budget has quadrupled from $50 million to $200 million in the last 10 years.

Critics call the spending a boondoggle. For example, the government pays Great Lakes Airlines to provide service between Ely and Las Vegas, but there is so little demand that sometimes planes take off without a single passenger on board.

But others see the program as a critical financial lifeline to ensure economic stability in rural areas.