Ukraine on Monday began a series of large-scale air force exercises with the United States and other NATO countries, the defence ministry said.
The "Clear Sky 2018" war games, which will run until October 19, are being held in western Ukraine.
Some 700 troops are taking part, half of them from NATO member countries including the United States, Britain, the Netherlands, Poland and Romania.
US aircraft including F-15C Eagle fighter planes and C-130J Super Hercules military transport planes and drones will train with about 30 Ukrainian aircraft, the ministry said.
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.
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In recent years, concerns have risen about Russia's intentions in Ukraine, particularly after Moscow's annexation of the Crimean peninsula in 2014.
Kiev and the West have accused Russia of backing rebels in Ukraine's war-torn east and sending troops across the border, claims Moscow has repeatedly denied.
The conflict has already killed more than 10,000 people since April 2014.