Newer generation fighter jets are giving Russia "increasing dominance" in the skies over the conflict zone in eastern Ukraine, according to a senior Ukrainian official.
It is a potentially worrying shift for the United States and its allies because Russia’s inability to achieve total air superiority throughout the course of the war has been key to Ukrainian successes on the battlefield.
In a briefing last week with journalists, a Western official downplayed the concern, stating that Russia’s ability to control the skies has been limited by air defense and surface-to-air missiles.
"We’re not seeing a huge change in that situation," the official said.
The senior official in Kyiv, who spoke exclusively to ABC News, said Russia had replaced older models of aircraft with its more modern Su-35.
The Su-35 is equipped "with very effective radar and long-distance rockets," the official said, and Russia is using these aircraft to attack Ukrainian jets in the air as well as for ground support operations.
Ukraine "does not have capabilities" to counter this threat, the official added.
"[Russian] air superiority is a real risk," the official said, adding that more air defense systems from the United States and its allies is currently "priority No. 1" for Ukraine.
Two other Ukrainian officials confirmed that Russian dominance of the skies over the combat zone was a top issue, with one official claiming Russia has "12 times more aircraft" than Ukraine.
"This is a problem," one official said. "What we keep telling the Americans is that in the end, there is no other solution than to give us [Western] fighter jets."
The Biden administration has so far resisted calls to supply Ukraine with F-16s.
Asked if he was open to the idea, President Biden said "no" in January.
Discussions are ongoing between Ukraine and NATO countries about the supply of modern fighter aircraft.
Two Ukrainian pilots have been taking part in an assessment in Arizona on flight simulators in order to ascertain how much training they would need if they were to adapt to Western fighter jets.
Retired Col. Steve Ganyard, who is also an ABC News contributor, said he believes Russia’s air superiority may largely come down to numbers.
"Sometimes quantity can be better than quality," Ganyard said. "Being short on aircraft and anti-aircraft systems is putting Ukraine at increasing disadvantage."
Last week Slovakia said it had handed over the first four of 11 Soviet-era Mig fighter jets which it had promised to Ukraine.
Poland has also promised to supply a number of Mig fighter jets to Ukraine.
On a trip to London last month, Ukrainian President Volodymyr Zelenskyy made a very public plea for Western fighter jets.
The senior Ukrainian official who spoke to ABC News suggested there was now more of an acceptance in Kyiv that U.S. military aid should be focused on other priorities such as air defense systems and the provision of artillery ammunition.
The official recognized that the supply of modern fighter jets, as well as the missiles to go with them, would require "a huge amount of money" and it could only come from the United states at the expense of other types of military aid.