The U.S. has conducted an overflight of Ukraine, under the terms of a 16-year-old arms control treaty, aimed at sending a symbolic message to Moscow, the Pentagon said Thursday.

“Today, the United States and Allies conducted an extraordinary flight under the Open Skies Treaty,” said a statement, released on the Pentagon’s website. “The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations.”

The description of the flight as “extraordinary” refers to the fact it was not previous scheduled or announced, an official said.

The plane, a U.S. Air Force OC-135 observation plane, did not fly over Crimea or any contested or Russian-controlled areas of Ukraine, the official confirmed.

The overflight was requested under the terms of the 2002 Open Skies Treaty by the Arms Control Directorate of the Ukrainian General Staff, said Eric Pahon, a Pentagon spokesman.

On board were observers from the U.S., Canada, Germany, France, the United Kingdom, Romania, and Ukraine.

In its statement, the Pentagon said the U.S. remains “resolute in our support for the security of European nations,” and it called Russia's seizure of Ukrainian naval vessels in the Black Sea near the Kerch Strait “a dangerous escalation in a pattern of increasingly provocative and threatening activity.”

The Open Skies flight, with 25 U.S. military personnel on board, departed Joint Base Andrews on Nov. 30, traveled to Ramstein Air Base, Germany, and flew over Ukraine Thursday.

“The United States seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere,” the Pentagon said.

The 2002 Treaty on Open Skies establishes a regime of flights over the territories of its signatories and is designed to promote openness and transparency in military activities through reciprocal unarmed observation flights.