WASHINGTON: A US Air Force observation plane flew over Ukraine on Thursday in a mission to show US support for Kiev following Russia's seizure of three Ukrainian naval vessels in the Sea of Azov, officials said.

The November 25 confrontation was the first open military incident between Kiev and Moscow since 2014 when Russia annexed the Crimean peninsula and a conflict erupted in eastern Ukraine.

"Today, the United States and Allies conducted an extraordinary flight under the Open Skies Treaty. The timing of this flight is intended to reaffirm US commitment to Ukraine and other partner nations," the Pentagon said in a statement.

The Open Skies Treaty is aimed at promoting military transparency through reciprocal, unarmed observation flights over each of the 34 signatory countries' terrains.

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.

This was the first "extraordinary" flight under the Open Skies Treaty since 2014, the Pentagon said. Such flights occur outside the schedule of normal, planned operations when unusual situations arise.

After last month's incident, Russia took 24 Ukrainian seamen into custody. The sailors have been put before a court in Simferopol, the main city in Russian-annexed Crimea, and ordered to be held in pre-trial detention for two months.

"Russia's unprovoked attack on Ukrainian naval vessels in the Black Sea near the Kerch Strait is a dangerous escalation in a pattern of increasingly provocative and threatening activity," the Pentagon said.

"The United States seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere."