The US Air Force transferred KS-135 Stratotankers from the RAF Mildenhall base in the UK, the McConnell AFB in Kansas, the Beale AFB in California and the US Air National Guard base in Illinois. They were accompanied by about 150 personnel.
A USAF press release said the aircraft were sent to Ukraine as part of the US strategy to "defend European Allies, enhance security in Eastern Europe and increase the level of military understanding between Allies and partners."
The Air Force also boasted that the deployment would help "further integrate capabilities, enhance interoperability, and ensure the security and stability of the region."
A Ukrainian military aviation enthusiast photographed the planes on the runway in Lviv.
"You search for fresh photos of the US Air Force, and suddenly 4 KC-135 tankers turn up in one Ukrainian city."
The aircraft are being used in ongoing military drills, which started on June 4 and set to wrap up on June 16.
Military cooperation between Ukraine, the US and its NATO allies intensified after the Maidan coup d'état in February 2014. The alliance has provided the Ukrainian military with training assistance and hundreds of millions of dollars of equipment, ranging from armored Humvees and drones to radar systems, sniper rifles and Javelin anti-tank systems. Last month alone it was reported that Ukraine had received over $40 million worth of equipment from the US, the UK, Canada and Lithuania.
Moscow has expressed concerns over NATO's ongoing buildup along its western borders. In the spring, during a meeting with the Russian Security Council, President Vladimir Putin accused the alliance of hindering integration processes involving Russia, fueling old conflicts and provoking new ones in the post-Soviet space and other regions bordering Russia. Russian officials have also repeatedly pointed out that despite promises made to Moscow at the end of the Cold War not to expand east, NATO, over the last two decades, has swallowed up every member of the former Warsaw Pact,, as well as the former Soviet republics of Lithuania, Latvia and Estonia.