Safety issues are on the rise at airlines in the United States, which are increasingly seeing their airplanes fall apart in the skies without warning or apparent cause.

Many of the reported problems started with Southwest Airlines and United Airlines but have since expanded to include American Airlines, a pilot group from which says it is seeing a "significant spike" in both safety- and maintenance-related problems at the carrier.

The Allied Pilots Association (APA) reported "problematic trends" in an April 13 memo to the airline, including instances of hammers and other tools being left in airplane wheel wells, as well as an increasing number of collisions between aircraft while they are being towed across the runways.

Roughly 15,000 pilots are part of the APA, which continues to implore its members to take their time in doing their jobs and not rush due to pressure because doing so puts both crew and passengers at risk of injury or death.

Alaska, United both seeing problems as well

Another air carrier seeing problems is Alaska Airlines, which back in January had a large panel blow off the side of one of its aircraft mid-flight. United Airlines also found itself in headlines after a wheel fell off a plane after takeoff, as well as an incident in which an aircraft skidded off an airport runway.

"While United Airlines is currently under public and government scrutiny, it could just as easily be American Airlines," the APA wrote in its memo.