Ukraine will shortly receive the first of its long-awaited Western-made fighter jets as the U.S. Defense Department comes under new heat to expand its training program for Ukrainian F-16 pilots.

On Monday, Kajsa Ollongren, Dutch outgoing defense minister, said that "the delivery of the first aircraft will take place soon." It is not clear how many jets will be part of the initial tranche, and when exactly they will arrive. A spokesperson for Ukraine's air force told Newsweek that this information was classified, and did not provide additional details.

The Netherlands has pledged a total of 24 F-16 jets to Ukraine as part of an international coalition with Denmark, Norway and Belgium to furnish Kyiv with advanced fighters. Ukraine will receive upward of 80 jets, a number that falls far short of the total number Kyiv has said it needs.

Ukraine's Western backers were reluctant to pledge the Lockheed Martin-made jets, which were a much-larger commitment than equipment such as tanks or artillery systems that featured in military aid packages throughout the war.

The jets will be a significant upgrade to Ukraine's air force, battered by more than two years of war against the Russians' superior and larger fleet. Russia has used its aircraft, flying outside of the reach of Kyiv's air defenses, to launch devastating aerial guided bomb strikes on Ukraine.

However, uncertainty has persisted about just when Kyiv will get hold of F-16s and have the aircraft operational against Russian forces, the timelines plagued by vagueness and delays.

Before the jets take to the skies, F-16 donor countries say, Ukrainian personnel must complete their training programs, and the necessary infrastructure and facilities must be up and running in the country.

Tension has becoming increasingly visible between Kyiv and its backers over training schedules. A batch of 10 Ukrainian pilots completed basic flying, ground school and language training with the British air force in March 2024, according to the U.K. government. It added the Ukrainians would be moving on to "advanced flying training" with the French air force. This is already underway.

A handful of Ukrainian pilots are also receiving training with the U.S. military at Tucson, Arizona, and at a Danish air base. Another training hub is being readied for Ukrainian pilots in southeast Romania.

Oleksandra Ustinova, a Ukrainian politician heading up Ukraine's arms and munitions commission, told U.K. newspaper The Times last month that the U.S. was making "excuses" for too few pilots being trained to fly F-16s. Ukraine will have 20 fully-trained pilots by the end of 2024, Ustinova said last month, adding in remarks to Ukrainian media that the training of personnel was the biggest problem.

Politico reported in mid-June that Kyiv officials were pushing the U.S. and other partner countries to increase the number of F-16 pilots being trained, but the Arizona-based facility, the Danish program and the Romanian base had limited training spots available.

The outlet then reported on Monday that a group of Democrats in the House of Representatives were pushing for an additional 10 Ukrainian F-16 pilots to receive training in the U.S. by the end of the year.

"By the end of the year, Ukraine will have more F-16 aircraft than they will have qualified pilots to fly them. This situation threatens to undermine the strategic advantages these aircraft could provide Ukraine," a collection of 15 House Democrats said in a letter obtained by Politico.