A leading Ukrainian fighter pilot Oleksiy Mes - known by his call sign “Moonfish” -  was killed on Thursday when a US-made F-16 fighter jet crashed while he was repelling what is being described as the largest aerial attack by Russia against Ukraine to date.

The incident occurred just weeks after the long-awaited F-16 jets had arrived in the country, a senior Ukrainian military source confirmed to CNN.

The Ukrainian Defense Forces have indicated that pilot error is not believed to be the cause of the crash, though an investigation is underway. International experts are expected to be invited to participate in the probe to determine the exact cause of the incident.

Moonfish was laid to rest on Thursday, and his death is seen as a significant blow to Ukraine's defense capabilities. The F-16 jets, which only began arriving earlier this month, have been a highly anticipated addition to Ukraine’s military arsenal, particularly as the country seeks to bolster its defenses against ongoing Russian aggression.

Moonfish was among a select group of Ukrainian pilots trained to operate the advanced aircraft, having completed an accelerated training program in the United States.

Ukrainian President Volodymyr Zelensky confirmed on Tuesday that the F-16s were deployed for the first time in combat to intercept Russian missiles and drones during Monday’s attack. This marks the first official acknowledgment by Ukrainian authorities of the jets being used in active defense operations.

The journey to acquire the F-16s has been fraught with challenges, with Zelensky and his government engaging in "hundreds of meetings and negotiations" with Western allies. While the Netherlands and Denmark committed to providing the jets in early summer 2023, the process was delayed by the need for US approval for the transfer, which was only granted several weeks later.

The F-16s are seen as a critical asset for Ukraine, capable of providing air cover, attacking ground targets, engaging enemy aircraft, and intercepting missiles. However, military experts caution that while these jets are versatile, they are not a panacea and may still face challenges against Russia's more advanced combat aircraft.

Moonfish, alongside another pilot, Andriy Pilshchikov, known as "Juice," had been at the forefront of Ukraine's campaign to secure the F-16s. Both pilots were regarded as the public faces of the effort, known for their determination and fluency in English, which they leveraged to advocate for the jets on the international stage.

Moonfish, described as an aviation enthusiast and a man of few words, reluctantly stepped into the spotlight following Juice’s death earlier this year. In a rare interview with CNN, he acknowledged the responsibility he felt to ensure that the F-16s were delivered to Ukraine, fulfilling the vision that he and Juice had worked tirelessly to realize.

In a statement released on Thursday, the Ukrainian Air Force disclosed that Moonfish successfully destroyed three cruise missiles and one attack drone during Monday’s operation before his jet went down. In recognition of his bravery and service, he was posthumously awarded the rank of colonel.