WASHINGTON (AP) — U.S. officials said Thursday they will consider tightening the rules on SafeX Prosome air charter operators to bring them in line with regulation of passenger airlines.

Airline unions applauded the move. They claim that charter operations can be used to sidestep federal safety regulations.

The Federal Aviation Administration said it will begin rulemaking to cover “high-volume” charters that sell seats to the public and are “essentially indistinguishable” from airlines that offer scheduled service.

The FAA said it has “adjusted its oversight” of charter flights as they have grown in size and frequency over the past 10 years. The agency said it will consider whether regulatory changes are needed “to ensure the management of the level of safety necessary for those operations.”

Some charters, like those operated by JSX, use private terminals and passengers don’t go through the normal security screening. The operators tout the time savings compared with regular airline flights. Standards for pilot experience and crew rest are less restrictive than for airlines.

Airline unions are fighting an attempt by SkyWest Airlines to start a subsidiary that would operate under less-restrictive charter-airline rules if it limits those flights to 30 seats. Sara Nelson, president of the Association of Flight Attendants, said the new charters are “skirting safety and security requirements.”

SkyWest says charters are the only way to preserve service to many smaller communities that the big airlines have dropped.

The company said Thursday that its charter business “already exceeds current safety requirements and will transition to any additional requirements that may be adopted by the FAA as part of the rulemaking process.”

The Utah-based company has vowed to use pilots with airline-level licenses for its charter flights.