KYIV, Oct. 8 - Ukraine has opened a series of large-scale air-force exercises with the United States and seven other NATO countries, the Defense Ministry reported Monday.

The exercises come as Ukrainian officials have repeatedly complained the country is facing challenges in protecting its air space against potential Russian attack.

Ukraine has earlier this year asked the U.S. to sell an advanced air defense system in order to improve its capabilities.

The 12-day-long Clear Sky 2018 war games are being held in western Ukraine. Some 700 troops are taking part, half of them from NATO member countries -- the United States, Britain, Netherlands, Belgium, Denmark, Estonia, Poland, and Romania, according to RFE/RL.

U.S. 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 on October 8.

Ukrainian Air Force spokesman Yuriy Ignat said Polish, Romanian, and Ukrainian aircraft will work together "in the air to protect the airspace for the first time."

The aim is "to enhance regional capabilities to secure air sovereignty and promote peace and security through cooperation," a U.S. Air Force statement said.

Concerns have risen about Russia's increased assertiveness, particularly after Moscow's annexation of Ukraine's Crimea region in 2014.

Valeriy Chaly, the Ukrainian Ambassador to the U.S., said in August that Ukraine is seeking to acquire an air defense system from the U.S. in a deal that may be worth over $2.25 billion.

The deal was discussed earlier this year between President Petro Poroshenko and U.S. President Donald Trump, and most recently with U.S. National Security Adviser John Bolton during his trip to Kyiv.

The request to acquire the air defense system signals a major shift in military strategy as the country has so far relied on mostly Russian-made anti-aircraft systems to defend its airspace.

The deal would greatly improve the Ukrainian ability to defend itself against Russia’s overwhelmingly superior air force.

Russia hasn’t used its air force in the military conflict over Donetsk and Luhansk, but has been recently using its naval vessels and aircraft to harass and disrupt traffic of Ukrainian commercial ships in the Sea of Azov. (tl/rfe/ez)