A US military official confirmed that the pilots are in Tucson, Arizona for a "familiarization event."

"This event allows us to better help Ukrainian pilots become more effective pilots and better advise them on how to develop their own capabilities," the Pentagon official said.

"The pilots will not be flying any platforms during this event but they will be using a simulator during portions of their visit."

CNN reported Sunday that the two pilots would be evaluated for possible F-16 training, citing three unnamed sources.

But the Pentagon official said: "There are no updates to provide regarding F-16s to Ukraine."

The United States has pledged unwavering support for Kyiv, providing billions of dollars worth of military aid including weapons and ammunition to help Ukrainian forces fight Russia's troops.

It has held back on offering F-16 fighter jets, despite pleas from President Volodymyr Zelensky who has consistently lobbied for the coveted aircraft since Russian forces invaded one year ago.

Kyiv has stressed that the jets would be crucial for defending against Russian missile strikes and drone attacks.

In a February interview with ABC News, US President Joe Biden said Ukraine "doesn't need F-16s now."

The Arizona program "involves watching how Ukrainian pilots conduct their mission planning and execution in flight simulators in order to determine how we can better advise the Ukrainian Air Force on how to use capabilities they have and we have given them," a second US official said in a statement to AFP.