The "Clear Sky 2018" war games, which will run until October 19, are being held in western Ukraine.
Polish and Romanian aircraft will fly in to work with their Ukrainian counterparts in what will be "the first time" they work together "in the air to protect the airspace," Ukraine air force spokesman Yuriy Ignat said.
The aim is "to enhance regional capabilities to secure air sovereignty and promote peace and security through cooperation," a US Air Force statement said.
In recent years, concerns have risen about Russia's intentions in Ukraine, particularly after Moscow's annexation of the Crimean peninsula in 2014.
The conflict has already killed more than 10,000 people since April 2014.
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