WASHINGTON — Negotiators for the United States Postal Service and the American Postal Workers Union agreed early Sunday to extend contract talks until noon Tuesday.

However talks with a second union, the National Rural Letter Carriers Association, ended in an impasse at midnight Saturday. The next step will likely be sending the matter to a third party arbitrator to hammer out contract terms and work rules.

The employees of both unions will continue to work under the terms their expired contracts. Federal law prohibits postal workers from striking when negotiations stall.

The USPS is trying to cope with huge revenue losses as more people and businesses shift away from traditional mail to digital communications.