U.S. aviation regulators are warning airlines and other aviators that a new band of 5G mobile phone service might interfere with key safety devices on aircraft.

The Federal Aviation Administration on Tuesday issued a Special Airworthiness Information Bulletin warning that “action might be required to address potential interference with sensitive aircraft electronics.”

The 5G spectrum abuts radio signals used by so-called radar altimeters, which measure how close an aircraft is to the ground.

While FAA took pains to say it is working with other agencies to allow the new technology to safely coexist with aviation, the safety alert illustrates growing frustration within the aviation industry.

The U.S. government’s reaction also puts it at odds with other nations. Canada recently imposed restrictions on locating new 5G cell towers near the runways of large airports. Australia, France and other nations have taken steps to limit the chances of aircraft interference.

Radar altimeters are used on planes and helicopters for multiple critical safety functions, including landing when visibility is low, anti-collision warnings and systems that warn pilots when they inadvertently get too low. Some commercial helicopter flights can’t operate without a working radar altimeter.

The FAA bulletin said pilots should remind passengers to place any 5G device into airplane mode or switch them off during flight, and to notify the agency of any signs of interference.

The new 5G spectrum can become operational on Dec. 5. The Federal Communications Commission awarded wireless network providers access to the radio bands in a February auction.

The FCC and CTIA, a trade group representing the wireless industry, didn’t immediately respond to requests for comment. CTIA has said that the radio frequencies aren’t the ones used by radar altimeters and they shouldn’t cause interference if the aviation devices are properly designed.

RTCA, a Washington-based nonprofit that studies technical aviation issues, in a report last year concluded that the potential for interference created a safety hazard. It found “significant impacts throughout the approach with the potential for catastrophic effects.”

The FAA bulletin said there have been no confirmed reports of interference.

The issue is that the FAA’s technical standards for radar altimeters were crafted years before the potential for mobile-phone companies to use nearby frequencies arose. As a result, tens of thousands of the devices are in use without any protection against adjacent radio waves, the FAA said in the bulletin.

In comments to the FCC, aviation industry representatives have said that it would take years to develop new standards for radar altimeters and then replace or upgrade them.