The U.S. military and several allies took part in “an extraordinary flight” over Ukraine under the Open Skies Treaty that was meant to send a message to an increasingly aggressive Russia, the Pentagon said in a statement Thursday.

“The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations,” according to the statement. “The United States is resolute in our support for the security of European nations.”

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The statement did not say what kind of aircraft was used, where specifically it flew over, or how many allies were involved in the flight.

The State Department later wrote on Twitter that the U.S. military flew an Air Force OC-135 — an unarmed observation aircraft used as part of Open Skies — and that Canada, Germany, France, Romania and the United Kingdom also participated in the mission.

The 2002 Open Skies Treaty, signed by the United States and 33 other nations — including Russia — is an arms control agreement that permits unarmed aerial recon flights above the territory of all signatory nations.

The United States has accused Russia of violating the treaty by restricting U.S. surveillance flights over the militarized region of Kaliningrad.

Tensions with Moscow have been heightened in recent weeks following the country’s seizure of three Ukrainian ships late last month off the coast of Ukraine’s Crimean Peninsula. Russian vessels opened fire on the Ukrainian ships before seizing the three ships and 24 sailors.

Russia, which annexed Crimea in 2014 to international condemnation, said the boats were operating unlawfully in its territorial waters, and that they were provoked into firing when the Ukrainian ships began behaving aggressively.

“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,” the statement said.



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