WASHINGTON (AP) — An unarmed U.S. Air Force plane with American and international observers flew a special flight over Ukraine Thursday in a show of solidarity after Ukraine's naval confrontation last month with Russia, the Pentagon said.

In a statement, the Pentagon called it an "extraordinary" flight under the Open Skies Treaty, which is an international accord that gives each member country the right to conduct, and the obligation to accept, unarmed military observation flights. The purpose to is promote transparency in military activities.

Normally these flights are arranged well in advance. But the treaty also permits "extraordinary," or extra, flights if two participating members agree. In this case, the Ukrainian military requested a flight and the U.S. agreed to conduct it over an uncontested portion of Ukraine territory, according to Eric Pahon, a Pentagon spokesman.

Pahon said this was the first "extraordinary" Open Skies flight since 2014 in response to Russia's annexation of Crimea.

On Nov. 25, Russia and Ukraine had a naval clash near Kerch Strait. Since then, Washington has sought to show its support for Ukraine while condemning Russian actions.

"The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations," the Pentagon said. "Russia's unprovoked attack on Ukrainian naval vessels in the Black Sea near the Kerch Strait is a dangerous escalation in a pattern of increasingly provocative and threatening activity," it added. "The United States seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere."

Pahon said the plane that was flown over Ukraine on Thursday was an OC-135 observation aircraft with U.S., Canadian, German, French, British, Romanian and Ukrainian observers aboard. He said the flight was requested by the arms control directorate of the Ukrainian General Staff.

Pahon said the plane had 17-member crew from the 55th Wing based at Offutt Air Force Base in Nebraska. It flew from Andrews Air Force Base in Maryland to Ramstein air base in Germany on Nov. 30 and flew over Ukraine on Thursday, he said.