The Pentagon, in a statement, called it "an extraordinary flight under the Open Skies Treaty", Xinhua news agency reported.
The Treaty on Open Skies was signed in March 1992 and became one of the major confidence-building measures in Europe after the Cold War. It took effect in 2002 and currently has 34 state parties, including Russia and most members of the North Atlantic Treaty Organization (NATO).
The Pentagon emphasized that "the timing of this flight is intended to reaffirm US commitment and support to Ukraine and other partner nations".
On November 25, three Ukrainian naval ships and more than 20 sailors attempting to sail through the Kerch Strait from the Black Sea to the Sea of Azov were seized by Russian forces for allegedly violating the Russian border.
The Ukrainian Navy called the incident an "act of aggression", saying it had informed Moscow in advance of the passage. Meanwhile, Russia said it had received no such report and the ships ignored multiple warnings by the Russian border guards.
The White House had earlier cancelled a planned meeting between US President Donald Trump and his Russian counterpart Vladimir Putin during the Group of 20 summit last weekend due to Russia's confrontation with Ukraine.
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