(CNN)The US is working with Ukrainian pilots in the United States to determine how long it would take to train them to fly F-16 fighter jets, three sources briefed on the matter told CNN.
Two Ukrainian pilots are currently at a military base in the US having their skills tested in flight simulators to see how much time they would need to learn to fly various US military aircraft, including F-16s.
A US military official told reporters traveling with Defense Secretary Lloyd Austin to the Middle East that the Ukrainian pilots were in Tucson, Arizona, for "a familiarization event" that he described as a "routine activity as part of our military-to-military dialogue with Ukraine."
"The familiarization event is essentially a discussion between the Air Force personnel and an observation of how the US Air Force operates," the official said.
"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 pilots will not be flying any platforms during this event but they will be using a simulator during portions of their visit."
The official added that "there are no updates to provide regarding F-16s to Ukraine" and there are no immediate plans to increase the number of Ukrainian pilots in the US.
While there is no indication at this time that flight training is underway, the step suggests the US has not completely closed the door on providing F-16s.
NBC News first reported the assessment of Ukrainian pilots.
The evaluation of the Ukrainian pilots is being conducted by the 162nd Fighter Wing, part of the Arizona Air National Guard which specializes in training on F-16 fighter jets, two US officials said Monday.
The 162nd Fighter Wing, which flies F-16 fighter jets, is based at Tucson International Airport, and a crucial part of its mission is training international aircrews on the fourth-generation aircraft. According to the Wing's website, it has trained pilots from 25 countries which currently operate the F-16 and describes itself as the "face of the USAF to the world."
One of the officials told CNN that 10 more Ukrainian pilots may come to the US for the same evaluation and assessment.
Ukraine has been pushing for the US to provide fighter jets and top Ukrainian officials have escalated their public lobbying campaign in recent days, arguing that they need the planes to defend against Russian missile and drone attacks.
But that push has been met with skepticism by US and allied officials, who say the jets would be impractical because they require considerable training and Russia has extensive anti-aircraft systems that could easily shoot them down.
When previously asked if the US would be providing F-16s to Ukraine, President Joe Biden responded with a flat "no."
Early on in the war, the US also believed that supplying Ukraine with new fighter jets would risk an escalation between NATO and Russia.
In an interview with ABC News in February, Biden said that Ukraine "doesn't need F-16s now," despite repeated public pleas from Ukrainian officials and mounting pressure from congressional Republicans.
US and European officials have previously told CNN that F-16 fighter jets were impractical in this situation.
Germany last week ruled out fighter jet deliveries to Ukraine completely while UK government officials echoed the sentiment and said that they believed it was not practical to send jets into Ukraine.
Dutch and Polish leaders also appeared reluctant when asked about sending F-16 fighter jets.
On Tuesday, Under Secretary of Defense for Policy Colin Kahl told the House Armed Services Committee that it would take 18 months to train Ukrainian pilots to fly F-16s and argued training pilots for that amount of time would be prohibitively expensive.
"It's just hard for me to tell any member of Congress or the American people that the best use of that dollar spent right now is on F-16s," Kahl said in the hearing.