Reuters reports of the latest instance: "The United States could impose sanctions on Egypt and block it from future military sales if it goes ahead with a purchase of Russian warplanes, a U.S. official said on Monday."

This after Egypt signed an agreement to purchase more than 20 Su-35 fighter jets from the Kremlin in a $2 billion deal.

The US has previously threatened other countries with sanctions under its Countering America's Adversaries Through Sanctions Act (CAATSA) for purchases of Russian military equipment (Serbia being the latest, related to rumors that it's mulling S-400 anti-air acquirement).

Cairo eyeing the advanced Russian fighter might be viewed as an especially insulting affront by the Trump administration especially given that as the second largest recipient of foreign aid historically (after Israel), Washington has provided billions in economic and military aid to Egypt over the past years, including its F-16 fighters.