The Open Skies Treaty is aimed at promoting military transparency through reciprocal, unarmed observation flights over each of the 34 signatory countries' terrains.
Ukraine responded by introducing martial law for 30 days, a measure Kiev did not take even after Crimea's annexation and amid large-scale fighting between Ukrainian forces and Russia-backed separatists in 2014-2015.
Normally these flights are scheduled well in advance.
On Tuesday, a senior U.S. State Department official warned that Moscow must release the Ukrainian sailors or face growing consequences.
The Kerch Strait is the only passage to and from the Black Sea and the inland Sea of Azov, which is jointly controlled by Russian Federation and Ukraine. It kicks off a 15-day wait before the ship can move into the Black Sea.
Arsenal to speak to players involved in 'hippy crack' controversy
Nitrous oxide is not illegal in Britain but experts say recreational use can be unsafe , especially if combined with alcohol. It is not illegal to possess nitrous oxide but it is illegal to supply it under the 2016 Psychoactive Substances Act.
Ukraine is accusing Russian Federation of keeping up its blockade of the Kerch Strait that links the Black Sea with the Sea of Azov, warning that it will soon send more navy ships through the strait.
The military added that "Russia's unprovoked attack" on Ukrainian ships near the Kerch Strait on November 25 is "a risky escalation in a pattern of increasingly provocative and threatening activity". Since then, Washington has sought to show its support for Ukraine while condemning Russian actions.
The US Air Force OC-135 observation plane was flown near Crimea, the Ukrainian region seized by Russian Federation in 2014, the Pentagon said. He said the flight was requested by the arms control directorate of the Ukrainian General Staff.
The United States has begun making preparations to sail a warship into the Black Sea amid Russia-Ukraine tensions, CNN has reported.
"A U.S. Air Force OC-135 observation plane flew over Ukraine today with U.S., Canadian, German, French, United Kingdom, Romanian and Ukrainian observers aboard", Pentagon spokesman Eric Pahon told VOA.
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