DALLAS » A union group is suing American Airlines, claiming supervisors are pressuring mechanics to release planes before they are safe to fly.

Local 591 of the Transport Workers Union says that union officials who raised objections were threatened with termination or even arrest.

American Airlines denies the allegations, saying that it complies with federal safety rules. A spokesman said Friday that regulators have not contacted the airline about any critical issues.

A spokesman for the Federal Aviation Administration declined to comment because the matter is under investigation.

The local and four union officials filed a lawsuit Thursday in federal court in Chicago. They said that airline managers pressured mechanics to disregard hydraulic leaks and wiring issues and to skip inspections after planes were hit by lightning or birds.

The Local 591 officials said mechanics faced retaliation for reporting safety violations including missing equipment, improper maintenance on a cabin door seal and cracked engine pylons.

According to the lawsuit, six mechanics raised the allegations in October in a whistle-blower complaint to the Department of Labor.

Airline spokesman Casey Norton said American works with the FAA "so that American’s maintenance programs, practices, procedures, and overall compliance and safety are second to none." He said the FAA team that oversees American has not alerted the airline "to any current critical issues or concerns."

FAA spokesman Lynn Lunsford said, "We are aware of allegations, but it’s under investigation and we can’t comment further."

Regulators have investigated American several times over maintenance issues. In 2008, the FAA forced American to ground all its MD-80 aircraft to fix improperly installed wiring. In 2013, American agreed to pay $24.9 million to settle FAA cases with potential fines totaling $162 million.

Local 591 and the four union officials are seeking unspecified damages. The local’s parent, the Transport Workers Union, represents mechanics and other ground workers at the airline, a unit of Fort Worth, Texas-based American Airlines Group Inc.

American has long had difficult relations with its labor unions, and the unions have struggled with internal divisions. Local 591’s president, Gary Peterson — one of the plaintiffs in the lawsuit against American — sued his parent union, TWU, in 2013 over who should represent the ground workers. Peterson’s case was thrown out.