The United States says it has carried out an "extraordinary" flight over Ukraine under an international military surveillance treaty, amid what it called a pattern of “increasingly provocative and threatening activity" by Russia.
The Pentagon said the flight took place on December 6, with tensions soaring between Ukraine and Russia following a naval confrontation last month.
CNN has reported that the United States is also making plans to sail a warship into the Black Sea, but that has not been confirmed.
Ukrainian government forces have been fighting against Russia-backed separatists in eastern Ukraine since April 2014, shortly after Russia military forces seized Ukraine's Crimean Peninsula and illegally annexed the territory.
Tensions between Kyiv and Moscow escalated on November 25, when Russia fired on Ukrainian naval vessels that were attempting to pass under a massive bridge that spans the Kerch Strait and links Russia with Crimea.
Russia ultimately seized three Ukrainian ships and 24 crewmen, who remain in Russian captivity despite international calls to free them.
“Today, the United States and allies conducted an extraordinary flight” under the Open Skies Treaty to “reaffirm U.S. commitment to Ukraine and other partner nations,” the Defense Department said in a statement.
Since 2002, the Open Skies Treaty has allowed 34 signatory states to send unarmed observation flights over one another's territory.
These flights are usually scheduled well in advance, but the treaty allows "extraordinary," or unscheduled flights, if two participating members agree -- in this case Ukraine and the United States.
Russia's “unprovoked attack” on Ukrainian vessels in the Black Sea is “a dangerous escalation in a pattern of increasingly provocative and threatening activity,” the Pentagon also said.
It added that the United States “seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere.”
U.S. surveillance planes and drones regularly skirt Russia's Black Sea coastline, as well as that of Crimea. Russian jets have been shown regularly confronting, and shadowing, the surveillance planes.
CNN reported on December 5 that U.S. military officials have asked the State Department to notify Turkey of possible plans for a Navy ship to pass through the Bosphorus and the Dardanelles, the waterway that connects the Mediterranean to the Black Sea.
The channel cited three unnamed U.S. officials as the source for the report, which could not be immediately confirmed.
A decades-old treaty requires that all countries that do not have a Black Sea coastline give Turkey 15-days notice when their warships plan to transit the waterway.
Two of the unnamed officials told CNN that the plans weren't set in stone, and that the notification was merely to provide the Navy with the option to move a warship into the area.
"We routinely conduct operations to advance security and stability throughout the U.S. 6th Fleet area of operations to include the international waters and airspace of the Black Sea," said Commander Kyle Raines, a spokesman for the U.S. Sixth Fleet, which oversees naval operations in the region.
"We reserve the right to operate freely in accordance with international laws and norms," he added.
Konstantin Kosachev, who heads the international affairs committee in Russia’s upper house of parliament, the Federation Council, said the U.S. military presence in the Black Sea will only increase tension in the region, according to the TASS news agency.
Viktor Bondarev, the chairman of the Federation Council’s committee on defense and security, called the possible deployment of U.S. Navy ships in the Black Sea as “yet another episode of saber rattling, yet another ploy to humiliate Russia," TASS reported.
NATO, which includes several Black Sea member states, and the United States have routinely sent warships patrolling the Black Sea, despite Russian concerns.
After the five-day Russia-Georgia war in 2008, with Russian forces still encamped on shore nearby, Washington anchored the flagship for the U.S. Sixth Fleet -- the USS Mount Whitney -- off Georgia's coast in a sign of support for the country.
Under a 2003 treaty, Russia and Ukraine agreed to share access to the Sea of Azov. However, since the 2014 annexation, and the completion of the Kerch bridge earlier this year, Russia has slowly restricted access for Ukrainian ships.
According to CNN, the last U.S. ship to enter the Black Sea was the fast transport ship USNS Carson City in October.