(Adds details from the court decision)

June 14 (Reuters) - A U.S. federal court on Friday granted American Airlines Group Inc's request to halt an alleged illegal slowdown by its mechanics' unions that the airline said had become "devastating" to its operations.

The United States District Court for the Northern District of Texas granted American Airlines, the world's largest airline by passenger traffic, a temporary restraining order against the airline's two mechanics unions demanding the workers stop interfering in American's operations.

The restraining order followed a lawsuit and preliminary injunction order that American filed against the unions three weeks ago.

The unions have "dramatically expanded their illegal slowdown activities and are creating an operational crisis causing significant damage to American, the traveling public and American's employees," the company said in its request for the restraining order.

The District Court ordered the two unions, the Transport Workers Union of America and the International Association of Machinists, to issue a notice to their workers to no longer continue activities such as refusing to work overtime, slowing their job performance or other activities that would negatively affect American's operations, according to the court filing on Friday.

American alleged in the lawsuit filed in May that the unions are directing the slowdown to strengthen their position in labor talks that began in 2015.

The unions have complained that American is trying to outsource more maintenance jobs, a move American has indicated is necessary to cover increased wages.

The slowdown has resulted in 722 flight cancellations since the company filed the lawsuit, the airline said, adding that the number of affected passengers has increased to 11,000 per day since June 7. (Reporting by Soundarya J and Mekhla Raina in BENGALURU; Editing by James Emmanuel and Christian Schmollinger)