The United States is evaluating Ukrainian pilots for possible training on the F-16 fighters

(CNN) The United States is working with Ukrainian pilots in the United States to determine how long they can train to fly F-16 fighter jets, three sources familiar with the matter told CNN.

Two Ukrainian pilots are currently at a military base in the US where their skills are being tested on flight simulators to see how long it will take them to learn to fly various US military aircraft, including the F-16s.

There is no indication at this time that flight training is taking place. But this decision indicates that the United States has not completely closed the door to the supply of F-16s.

The sources indicated that it is possible that other Ukrainian pilots will come to the United States as early as this month.

They added that some lawmakers and their teams have been briefed on the assessment of the Ukrainian pilots.

Ukraine has been seeking to supply combat aircraft to the United States, and senior Ukrainian officials have stepped up their campaign of public pressure in recent days, saying they need the jets to defend against missile attacks and Russian drones.

But that push was met with skepticism from US and allied officials, who said the planes would be unwieldy because they require too much training and Russia has extensive anti-aircraft systems that it can easily shoot down.

When previously asked if the United States would supply the F-16s to Ukraine, President Joe Biden answered, “No.”

At the beginning of the war, the United States also believed that providing Ukraine with new combat aircraft would risk escalation between NATO and Russia.

In an interview with ABC News in February, Biden said Ukraine “doesn’t need the F-16s right now,” despite repeated public pleas from Ukrainian officials and growing pressure from Republicans in Congress.

US and European officials have previously told CNN that the F-16s are impractical in this situation.

Germany last week ruled out delivering fighter jets to Ukraine, while British government officials reiterated that sentiment and said they believed it was impractical to send planes to Ukraine.

The Dutch and Polish leaders also seemed hesitant 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 the F-16s, and he said training pilots during that time would be very expensive.

“It’s hard for me to tell any member of Congress or the American people that the best use of this dollar that is being spent right now is on F-16s,” Kahl said during the hearing.

NBC News first reported the assessment of the Ukrainian pilots.