Ukrainian Defense Minister Stepan Poltorak told reporters on Friday that his country intends to send naval ships through the Kerch Strait soon, saying that "otherwise Russian Federation will fully occupy the Sea of Azov".
This was the first "extraordinary" flight under the Open Skies Treaty since 2014, the Pentagon said.
"The timing of this flight is meant to reaffirm USA commitment to Ukraine and other partner nations", the Department of Defense said.
The Pentagon said the Ukrainian military had requested the flight, which was on an OC-135 surveillance plane and had observers from the US, Canada, Germany, France, Britain, Romania and Ukraine aboard.
In a statement, the Pentagon called it an "extraordinary" flight under the Open Skies Treaty, which is an worldwide accord that gives each member country the right to conduct, and the obligation to accept, unarmed military observation flights. The Russian vessels rammed and fired upon three Ukrainian ships before capturing the ships and their crews.
Ukraine's defense ministry warned Russian Federation on Friday that it will soon send navy ships through the Kerch Strait where Russian Federation fired on and seized three Ukrainian vessels two weeks ago.
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The flight, requested by the Arms Control Directorate of the Ukrainian General Staff, is the latest United States response to Russia's military activity across the world. The Open Skies is an observation aircraft used for treaty support enhancing global peace, stability and cooperative security.
This move aimed at "challenging Russia's excessive maritime claims" in a demonstrative "freedom of navigation" operation, they added.
A United States guided-missile destroyer was deployed to disputed waters off the Russian coast on Wednesday in a rare show of force not seen since the Cold War.
The U.S.is also making preparations to send a warship into the Black Sea, CNN reported Wednesday. Kyle Raines, a spokesman for 6th Fleet, told reporters.
"The United States seeks a better relationship with Russian Federation, but this can not happen while its unlawful and destabilising actions continue in Ukraine and elsewhere".