The United States has said it has conducted an “extraordinary” flight over Ukraine amid escalating tensions between Kiev and Moscow.

The announcement comes eleven days after a naval clash that saw Russian forces open fire on and seize three Ukrainian warships near the Kerch Strait.

In a statement released on Thursday, the Pentagon said the timing of the flight - undertaken as part of the Open Skies Treaty, was “intended to reaffirm US commitment to Ukraine and other nations.”

It described the “unprovoked attack” in Kerch as part of “a pattern of increasingly provocative and threatening activity.”

Although the Open Skies Treaty gives each of the 34 state parties the right to gather information about the military forces and activities of other state parties, an extraordinary flight allows for countries to conduct the flights on a bilateral basis - hence the action by the US and Ukraine.

It is the first such flight since 2014, which came as part of the US response to Moscow annexing Crimea from Ukraine.

The Defense Department said US, Canadian, UK, German, French, Romanian and Ukrainian observers were aboard the aircraft during the observation flight. This flight was requested by the Arms Control Directorate of the Ukrainian General Staff.

A total of 25 US military personnel took part in the flight, including eight Defense Threat Reduction Agency personnel and a 17-person air crew from the 55th Wing out of Offutt Air Force Base, Nebraska.

Washington is said to be making preparations to sail a warship into the Black Sea, although the option may not be taken up.

The flight also comes days after Washington said it suspending its participation in a Cold War-era nuclear arms treaty with Moscow over violations of the deal by the Kremlin. Russia denies any such violations.

Protests break out after Russia seize Ukraine warships in 2018 Show all 32 1 /32 Protests break out after Russia seize Ukraine warships in 2018 Protests break out after Russia seize Ukraine warships in 2018 Activists of opposition parties burn flares during a rally demanding to break an agreement with Russia on the use of the Azov Sea and the Kerch Strait, in front of the parliament building in Kiev Reuters Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps light flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev after Russia seized two of their armored artillery vessels and a tug boat in the Black Sea AP Protests break out after Russia seize Ukraine warships in 2018 Ukrainian Nationalists demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine EPA Protests break out after Russia seize Ukraine warships in 2018 Ukrainian activists burn flares during their rally in front of Russian Consulate in Kharkiv EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of opposition parties burn flares during a rally demanding to break an agreement with Russia on the use of the Azov Sea and the Kerch Strait, in front of the parliament building in Kiev, Ukraine November 26, 2018. REUTERS/Gleb Garanich GLEB GARANICH Reuters Protests break out after Russia seize Ukraine warships in 2018 Seized Ukrainian ships, small armoured artillery ships and a tug boat, are seen anchored in a port of Kerch Reuters Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps brandish flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" . (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 The President of Ukraine, Petro Poroshenko, announced, on 25 November, at a meeting of the National Security and Defense Council that they will introduce martial law in Ukraine for a period of 60 days and appeal to the Parliament to consider the move Reuters Protests break out after Russia seize Ukraine warships in 2018 Russia seized two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats. The 'Yany Kapu' tugboat has forcibly been stopped Reuters Protests break out after Russia seize Ukraine warships in 2018 Children from an orphanage volunteer to help the city defenders strengthen trenches on Ukraine's Army positions near the village of Rybatske 25 km from Mariupol AP Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps light flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 Policemen guard at Russian consulate during a protest action in the Black Sea Ukrainian city of Odessa AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 epa07190805 Ukrainian Nationalists burn flares and smoke grenades during their rally in front of Parliament building in Kiev, Ukraine, 26 November 2018 as they demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine. The President of Ukraine Petro Poroshenko at a meeting of the National Security and Defense Council on 25 November 2018 announced the decision to introduce martial law in Ukraine and to appeal to the Parliament to consider the move at an extraordinary meeting. It is about the introduction of martial law for a period of 60 days. The Ukrainian Parliament will convene for an extraordinary meeting at evening 26 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait on 25 November 2018. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. EPA/SERGEY DOLZHENKO EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of Ukrainian far-right groups shout slogans during their rally in front of the Ukrainian parliament in Kiev on November 26, 2018, as they demand to set martial law in the country and to cut diplomatic relations with Russia. - Ukraine's military was on high alert and parliament was to vote on November 26 on a request from President to impose martial law for 60 days after Russian navy seized three Ukrainian ships off the coast of Crimea. (Photo by Sergei SUPINSKY / AFP)SERGEI SUPINSKY/AFP/Getty Images AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 With relations still raw after Russia’s annexation of Crimea and its backing for a pro-Moscow insurgency in eastern Ukraine, the capture of three Ukrainian naval vessels risks pushing the two countries towards a wider conflict. Graphic explains how the naval incident unfolded on a map Graphic News Protests break out after Russia seize Ukraine warships in 2018 Activists of far-right parties burn flares in front of the parliament building in Kiev during a rally to support the Ukrainian navy Reuters Protests break out after Russia seize Ukraine warships in 2018 A Ukraine army APC moves toward on position at an undisclosed location in eastern Ukraine AP Protests break out after Russia seize Ukraine warships in 2018 Ukrainian Nationalists sign a banner with a slogan reading like 'Not retreat and not surrendering!' EPA Protests break out after Russia seize Ukraine warships in 2018 Activists of Ukrainian far-right groups hold flares during their rally in front of the Ukrainian parliament in Kiev on November 26, 2018, as they demand to set martial law in the country and to cut diplomatic relations with Russia. - Ukraine's military was on high alert and parliament was to vote on November 26 on a request from President to impose martial law for 60 days after Russian navy seized three Ukrainian ships off the coast of Crimea. (Photo by Sergei SUPINSKY / AFP)SERGEI SUPINSKY/AFP/Getty Images SERGEI SUPINSKY AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 Activists glue Ukrainian flag on the fence of Russian consulate AFP/Getty Protests break out after Russia seize Ukraine warships in 2018 A volunteer with the right-wing paramilitary Azov National Corps during a rally on the snowy streets in front of the Ukrainian parliament in Kiev AP Protests break out after Russia seize Ukraine warships in 2018 Volunteers with the right-wing paramilitary Azov National Corps hold flares during a rally on the snowy streets in front of the Ukrainian parliament in Kiev, Ukraine, Monday, Nov. 26, 2018. Some hundreds of protesters from far-right party National Corps brandished yellow-and-blue flags with the Ukrainian national trident symbol, and a banner reading 'Don't back down!" (AP Photo/Efrem Lukatsky) Efrem Lukatsky AP Protests break out after Russia seize Ukraine warships in 2018 epa07190891 Ukrainian Nationalists attend their rally in front of Parliament building in Kiev, Ukraine, 26 November 2018 as they demand to break the diplomatic relations with Russia and nationalization of Russian property in Ukraine. The President of Ukraine Petro Poroshenko at a meeting of the National Security and Defense Council on 25 November 2018 announced the decision to introduce martial law in Ukraine and to appeal to the Parliament to consider the move at an extraordinary meeting. It is about the introduction of martial law for a period of 60 days. The Ukrainian Parliament will convene for an extraordinary meeting at evening 26 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait on 25 November 2018. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. EPA/SERGEY DOLZHENKO SERGEY DOLZHENKO EPA Protests break out after Russia seize Ukraine warships in 2018 An activist piles up tires during a protest against the seizure by Russian special forces of three of the Ukrainian navy ships, which Russia blocked from passing through the Kerch Strait into the Sea of Azov in the Black Sea, in front of the Russian embassy in Kiev, Ukraine November 25, 2018. REUTERS/Gleb Garanich Reuters Protests break out after Russia seize Ukraine warships in 2018 A National Guard serviceman extinguishes a torch thrown by a protester during a rally against the seizure by Russian special forces of three of the Ukrainian navy ships Reuters Protests break out after Russia seize Ukraine warships in 2018 A protester throws a smoke grenade during a rally in front of the embassy of Russia in Kiev AP Protests break out after Russia seize Ukraine warships in 2018 Protestors set up paper ships on the tires during their rally near of Russian embassy building in Kiev EPA Protests break out after Russia seize Ukraine warships in 2018 Smoke from a flare thrown by a protester is seen during a rally against the seizure by Russian special forces of three of the Ukrainian navy ships, which Russia blocked from passing through the Kerch Strait into the Sea of Azov in the Black Sea, in front of the Russian embassy in Kiev, Ukraine November 25, 2018. REUTERS/Gleb Garanich GLEB GARANICH Reuters Protests break out after Russia seize Ukraine warships in 2018 A man extinguishes a burning car of the embassy of Russia Reuters Protests break out after Russia seize Ukraine warships in 2018 Paper boats are seen placed during a protest Reuters Protests break out after Russia seize Ukraine warships in 2018 epa07190225 Ukrainian policemen guard the Russian embassy building with smoke behind during Ukrainian activists rally in Kiev, Ukraine, 25 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait; Ukrainian President Petro Poroshenko is gathering the Military Cabinet over the incident. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. The Ukrainian Navy also reported the number of the Ukrainian servicemen wounded in the incident grew to two persons as Ukrainian media report. EPA/STEPAN FRANKO EPA-EFE/STEPAN FRANKO EPA Protests break out after Russia seize Ukraine warships in 2018 epa07190217 A protestor throws a smoke grenade at the building during their rally near the Russian embassy in Kiev, Ukraine, 25 November 2018. Russia has seized three Ukrainian vessels amid their leaving the Kerch Strait; Ukrainian President Petro Poroshenko is gathering the Military Cabinet over the incident. The two small-sized 'Berdiansk' and 'Nikopol' armored artillery boats have come under enemy fire and are now dead in the water. The 'Yany Kapu' tugboat has forcibly been stopped. The vessels have been captured by special forces of the Russian Federation, the press service of Ukraine's Navy said on Facebook on Sunday evening. The Ukrainian Navy also reported the number of the Ukrainian servicemen wounded in the incident grew to two persons as Ukrainian media report. EPA/STEPAN FRANKO EPA-EFE/STEPAN FRANKO EPA

Russia has arrested and charged all 24 sailors that formed part of the Ukrainian flotilla involved in the clash, the youngest of whom is 18. They all stand accused of illegally entering the territorial waters of Russia.

Distress signals sent by the Ukrainian ships suggest they were fired on when sailing away in international waters.

Under the terms of a 2003 agreement between Russia and Ukraine, signed before the annexation of Crimea, the Kerch Strait and Azov Sea are considered shared waters.

US suspends participation in Cold War-era nuclear treaty after Russia found in breach of agreement

Also on Thursday, Ukraine urged a gathering of dozens of foreign ministers to increase sanctions against Russia, accusing Moscow of sowing “instability and insecurity” across the West.

Speaking at a meeting of the Organisation for Security and Cooperation in Europe (OSCE) in Milan, Ukraine's foreign minister Pavlo Kilmkin denounced the seizure of the crew members.

“It is a matter of urgency to provide a prompt and consolidated international response to this act of aggression. Declarations are not enough. There must be action,” Mr Klimkin told the annual gathering.

“We must raise the cost for Russia with comprehensive and tailored sanctions," he added. "There can be no business as usual.”

However, Russian Foreign Minister Sergei Lavrov pinned the blame for years of escalating tensions on Ukraine and its allies - including the US.

“Striving for dominance, a small group of countries uses blackmail, pressure and threats,” Mr Lavrov said

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“Kiev (is) free from any punishment, shielded by its Western sponsors, who justify all its outrageous actions,” he added.

Meanwhile, the Ukrainian parliament has voted to withdraw from a wide-ranging treaty on friendship with Russia.

The Supreme Rada overwhelmingly backed a motion by President Petro Poroshenko not to prolong the treaty when it comes up for renewal in April. Mr Poroshenko also announced a new dispatch of troops to the Russian border during a visit to a barracks.