WASHINGTON (Reuters) - Prospects that Congress would enact a measure to overhaul U.S. aviation programs dimmed significantly on Wednesday as the House of Representatives moved to attach critical safety provisions to separate, must-pass spending legislation.

The action, by Transportation Committee Chairman James Oberstar and aviation subcommittee Chairman Jerry Costello, signaled that negotiators remain deadlocked on other proposals in the larger bill, with time to act dwindling.

House and Senate negotiators have so far failed to resolve key differences between separate bills previously approved by the chambers. These include airport fees, new service proposals and a provision that would make it easier for ground workers to unionize at FedEx Corp.

The House is scheduled to leave town on Friday for its August recess, while the Senate plans to do the same on August 6.

Oberstar said in a conference call that the House would vote on a proposal this week to continue funding of air traffic operations overseen by the Federal Aviation Administration (FAA) through September.

The measure must be approved to continue U.S. air traffic operations.

Oberstar said the short-term funding plan would include pilot training and other elements strengthening safety that were prompted by a Buffalo commuter air crash in 2009 that killed 49 people.

Those provisions had been a centerpiece of the now-stalled larger bill that proposed to fund the FAA for two years, authorize a downpayment on a $20 billion modernization of the aging air traffic control network, and raise passenger fees that fund airport capital projects.

The FedEx provision was not expected to survive, but disposing of it has raised procedural questions that have further delayed action.

While the temporary FAA funding measure headed to the full House would expire at the end of the fiscal year, the safety initiatives would become law. Oberstar said he had staff assurances that the Senate "was prepared to accept our proposal."

(Reporting by John Crawley; Editing by Steve Orlofsky)