A Ukrainian F-16, recently supplied by NATO countries, was destroyed this week while defending against a sweeping Russian aerial assault and the pilot was killed, Ukraine’s military said on Thursday.

The loss of the plane is a blow to the government in Kyiv, which received its first batch of F-16s this summer after arguing to the White House and allies in Europe for many months that they were essential to the war effort. While the United States and other allies have provided Ukraine with tens of billions of dollars of military equipment, the fighter jets have taken on outsized importance as a symbol of Western support.

The plane was lost after Ukraine scrambled its air defenses in an attempt to intercept more than 200 missiles and drones fired by Russia on Monday, in what President Volodymyr Zelensky of Ukraine said was one of the largest attacks of its kind since the war began 30 months ago.

The F-16s were involved in the effort and had “demonstrated their high efficiency,” shooting down four cruise missiles, Ukraine’s general staff said in a post on Facebook. But “communication was lost with one of the aircraft. It was later determined that the aircraft had crashed and the pilot was killed.”

The military said that it had opened an investigation into the cause of the crash. A Western official said the Ukrainians were looking at pilot error or mechanical failure, rather than enemy fire. The crash was reported Thursday by The Wall Street Journal.

Ukraine’s military general staff did not disclose the identity of the pilot who was killed, but the air force said that it had lost a pilot named Oleksiy Mes on Monday during the assault on the country.