Denmark, Norway, the Netherlands and Belgium plan to ship more than 60 U.S.-made F-16 jets to Kyiv this summer. Ukrainians argue that the ability to fly more F-16s will help Kyiv push back Russian forces from the frontlines in places such as Kharkiv, where Moscow has advanced in recent weeks. Yet the U.S. has said it does not expect the jets to be a game-changer on the battlefield.

This story is based on interviews with eight former and current U.S., Ukrainian and European officials and lawmakers, many of whom were granted anonymity to speak about sensitive diplomatic conversations.

In a series of meetings and calls over the last several weeks, Ukraine has officially requested the U.S. train the additional pilots at Morris Air National Guard base in Tucson, Arizona. Lawmakers on Capitol Hill, including Rep. Jim Himes (D-Conn.) and Rep. Mike Turner (R-Ohio), the leaders of the House Intelligence Committee, have also pressed the administration to approve the additional training. The pair, along with several other lawmakers, wrote the Pentagon a letter last month asking the administration to prioritize the issue.

But the U.S. has told the Ukrainian military that in addition to limited space, other countries are in line for F-16 training at the base and that it cannot break its commitments to those nations.

“We understand they don’t want to break those contracts, but they could move their American pilots to a different base for training,” said Sasha Ustinova, a Ukrainian lawmaker who has advocated for the training.

The National Guard is planning to train 12 Ukrainian pilots total by the end of September at the Tucson location, according to Air Force spokesperson Laurel Falls. Aside from Arizona, the training facility in Denmark also has limited space and is preparing to shutter in November. A third program, which is located in Romania and will be run by contractors, is not yet up and running.

“Dozens” of pilots from several countries are conducting basic flight training and F-16 training in the U.S. and Europe, said Pentagon spokesperson Maj. Charlie Dietz, declining to confirm specific numbers.

“We have developed and are implementing a training plan that meets forecasted aircraft fielding timelines, which are ultimately shaped by a number of factors, including maintenance and sustainment needs, infrastructure and support equipment requirements, and aircraft sourcing timelines, as well as pilot throughput,” Dietz said.

Maj. Erin Hannigan, a spokesperson for the Arizona National Guard, confirmed that space in the program is limited due to commitments to other countries’ training requests, funding and the completion by potential students of English language requirements.

“On top of the Ukrainian students, there are multiple other countries that have reserved training throughout the year,” Hannigan said. “The number of foreign student pilots and where they come from is not determined by our schoolhouse, there are many other factors that play a part in the numbers such as funding, country requests, graduation of students from English learning and allotment.”

The Air Force also trains F-16 pilots at Luke Air Force Base, Arizona, and Joint Base San Antonio, Texas, but those programs also have limited spaces for international students. Those spots are typically reserved well in advance for pilots from other countries that operate the F-16.

Another eight Ukrainian pilots are being trained in Denmark, according to a former DOD official involved in the program. But that facility is set to close next year and will no longer participate in the training, as Denmark’s Air Force transitions to the stealthy F-35.

Meanwhile, F-16-maker Lockheed Martin and its subcontractor, Draken, are also preparing to train pilots at a facility in Romania, but that program is expensive and also will have limited spots, according to the former official and a person familiar with the program. A Lockheed spokesperson referred questions about the Romania facility to the governments of the U.S., Romania and the Netherlands.

A total of 20 Ukrainian F-16 pilots are expected to graduate by the end of this year — half of the 40 needed to operate a full squadron of 20 jets, according to the former DOD official. Eight new pilots are scheduled to begin training in Romania, and eight more will soon arrive in Tucson, the former official said. The facility in Denmark will not accept any additional pilots.

Among the training facilities, only four slots will be open for Ukrainian pilots through the remainder of the year, according to the person familiar.

Meanwhile, U.S. officials have warned for months that the F-16 will not make a huge difference on the battlefield for Ukraine.

While the jets “will give the Ukrainians an increment of capability that they don’t have right now … it’s not going to be a dramatic game-changer as far as I’m concerned for their total military capabilities,” Air Force Secretary Frank Kendall said last year.

In the Kharkiv area, for example, one U.S. official said Ukraine will not be able to fly the jets to the border with Russia or into Russian territory, because Moscow’s air defense systems will easily spot them and shoot them down.

But officials involved in the program said the aircraft will still provide a significant advantage for Kyiv, eventually. When Ukraine initially gets its F-16s and pilots, they will likely be able to fly only limited missions, for example, against drones and cruise missiles on the front lines, the former DOD official said. Once they get a full squadron worth of aircraft and pilots, it’s “completely realistic” to fly the F-16s to the border and shoot into Russia given the jet’s radar, targeting system and missiles — all of which are superior to Ukraine’s Soviet-era fleet.

But at this rate, Ukraine won’t have a full squadron of trained pilots until the end of 2025, the former official said.

Another issue is the weapons the planes will be carrying.

Ukrainians plan to use the jets to knock down Russian cruise and ballistic missiles fired toward Ukrainian infrastructure and civilian targets.

Those missions require precise air-to-air missiles that belong to the U.S. and dozens of NATO allies. Many of those countries are hesitant to part with their expensive weapons, one NATO official said.

The production capacity for the AIM-120 advanced medium-range air-to-air missile, made by RTX, has increased from around 500-800 per year to over 1,000, company officials said last year, in order to keep with demand. Ukraine has already used an older version of the missile with the ground-based National Advanced Surface-to-Air Missile System to hit Russian missiles.

But the worry from capitals is that the new requests from Kyiv could begin to put a strain on countries’ stocks, and talks among alliance members are looking at who might be able to part with their missiles, and how many, and when, the official said.