American Airlines Group Inc. mechanics and their union have ignored a court order to end an alleged work slowdown, extending and worsening flight delays that have devastated its busy summer operation, the carrier said in a federal court filing.

The number of aircraft out of service at the start of each day remains above the average of the past two summers, even after a temporary restraining order was issued against the TWU-IAM Association on June 14, the airline said. American is asking a judge to issue more specific and stringent instructions for employees to return to normal work schedules immediately, under threat of “substantial financial sanctions” for both individuals and the union.

The late Wednesday filing ratchets up tension between American and the union, the only major labor group that hasn’t a new contract since the airline merged with US Airways in December 2013. Federal mediators suspended negotiations in April, saying they didn’t see a way to resolve differences. American says the slowdown is the union’s effort to gain leverage in the talks.

The TWU-IAM Association on Thursday declined to comment on American’s latest filing, saying it would respond in court. In an exchange of emails with American’s attorneys that was included in the documents, lawyers for the union “adamantly” disagreed with claims it hadn’t complied with the temporary restraining order.

American is seeking a permanent injunction against the union, with a one-day trial set for July 1. The airline earlier said the flight disruptions were affecting 11,000 passengers a day. The union represents 30,000 American employees, including mechanics, baggage handlers and other airport ramp workers.

The increased number of planes out of service at the start of each day has disrupted flights at all of American’s major hubs, with the biggest effect in Dallas-Fort Worth, Miami, Chicago and Philadelphia, the carrier said. The portion of assigned maintenance work completed each night fell to 69.5% after the restraining order was issued through June 24, compared with 75.7% last summer, according to the filing.