WASHINGTON — The U.S. has conducted an Open Skies flight over Ukrainian territory, in what the Pentagon is calling an explicit message to Russia that America remains committed to Ukraine’s defense.

“Today, the United States and Allies conducted an extraordinary flight under the Open Skies Treaty. The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations,” a release from the Pentagon announced Thursday. “The United States is resolute in our support for the security of European nations.”

The treaty allows the 34 signatory nations to fly over each other’s territory to verify military movements and conduct arms control measures; the U.S. operates the OC-135B, an aging airframe that has struggled with maintenance rates. This is the first flight under the treaty conducted by the U.S. in 2018.

The term “extraordinary” is a technical term; under the Open Skies treaty, each nation is given a specific allocation of flights it might request. However, an “extraordinary” request from a country for the U.S. to overfly its territory on short notice falls outside that allocation of flights.

The Sea of Azov won’t become the new South China Sea (and Russia knows it) Whereas China’s claims of maritime rights over most of the South China Sea have been rejected by an international tribunal, Russia’s rights in the Kerch Strait are well-defined.

In addition to U.S. personnel, Canadian, French, German, Romanian and British observers were on the plane, according to the U.S. State Department.

“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 release 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.”

The timing of the flight is notable for several reasons. First, it comes just hours after media reports said the U.S. Navy is preparing a “freedom of navigation” exercise through the Black Sea, near the Sea of Azov — a flashpoint between Ukraine and Russia since late November, when the latest crisis between those two countries kicked off.

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It also comes a day after the U.S. formally accused Russia of violating the Intermediate-Range Nuclear Forces Treaty and announced a 60-day warning period, after which the Trump administration said it will leave the cruise-missile control agreement with Russia if Vladimir Putin’s government does not fall back into compliance.

The flight is the first Open Skies flight conducted by America all year, in part due to an ongoing fight between the U.S. and Russia regarding what equipment may or may not be certified on the Russian Tu-214 aircraft. That fight seems to have been resolved in September.

Speaking to Defense News over the weekend, Andrea Thompson, undersecretary of state for arms control and international security, expressed her belief the Open Skies agreement would extend successfully into 2019.