(CNN) -- American Airlines has found cracks in large pylons that hold engines onto the wings of two of its 767-300 aircraft, according to the airline and to the planes' manufacturer.

Concerned the problem may exist on other airline fleets, Boeing says it plans to advise airlines to inspect the parts more frequently than the current recommendation of every 1,500 takeoff and landing cycles.

"This is considered a safety issue. You're talking about the strut that connects an engine to the wing," said Boeing spokesperson Peter Conte.

American Airlines spokesman Tim Wagner said the problem was first discovered on one of its 767-300s about two weeks ago during a maintenance check targeting other parts of the aircraft. The airline then found cracking on one more 767-300 after inspecting a total of 56 aircraft -- 767-300 and 767-200s -- a process American Airlines completed Monday night.

"We found these issues when it's best to find them, which is early on," said Wagner. The airline has sent the first damaged pylon to a metallurgy lab to try to determine why it cracked.

"Until that metallurgy testing is completed no one knows what the cause is," said Wagner. The damaged planes were among American's older aircraft, Wagner added.

American flies Boeing 767-300s on international routes as well as long-haul domestic routes. The 767-200s fly primarily transcontinental routes.

Airlines have previously found similar cracks at the holes where the pylon attaches to the wing, which led Boeing and the Federal Aviation Administration five years ago to recommend checks every 1,500 cycles. At that time, when the FAA issued an airworthiness directive it said, "We are issuing this (directive) to prevent fatigue cracking in the primary strut structure and reduced structural integrity of the strut, which could result in separation of the strut and engine."