The U.S. Department of Defense said on Thursday that the United States and its allies conducted an observation flight over Ukraine earlier that day.

The Pentagon, in a statement, called it "an extraordinary flight under the Open Skies Treaty."

The Treaty on Open Skies was signed in March 1992 and became one of the major confidence-building measures in Europe after the Cold War. It took effect in 2002 and currently has 34 state parties, including Russia and most members of the North Atlantic Treaty Organization (NATO).

The Pentagon emphasized that "the timing of this flight is intended to reaffirm U.S. commitment and support to Ukraine and other partner nations."

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

Speaking at a meeting of the Organization for Security and Cooperation in Europe (OSCE), the foreign minister of Ukraine denounced the recent seizure of three Ukrainian naval vessels and their crew by Russian forces off Crimea, saying it represented another assault on international law.

“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,” Pavlo Klimkin told the annual gathering of OSCE ministers.

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

The 57-nation OSCE, a security and human rights watchdog, has been rattled by the military and a diplomatic stand-off between Russia and Ukraine, both of whom are member states.

On November 25, three Ukrainian naval ships and more than 20 sailors attempting to sail through the Kerch Strait from the Black Sea to the Sea of Azov were seized by Russian forces for allegedly violating the Russian border.

The Ukrainian Navy called the incident an "act of aggression," saying it had informed Moscow in advance of the passage. Meanwhile, Russia said it had received no such report, and the ships ignored multiple warnings by the Russian border guards.

The White House had earlier canceled a planned meeting between U.S. President Donald Trump and his Russian counterpart Vladimir Putin during the Group of 20 summit last weekend due to Russia's confrontation with Ukraine.

The U.S. military has started preparations to dispatch a warship into the Black Sea amid the tension between Russia and Ukraine over the Kerch Strait incident, U.S. media reported on Wednesday.

(Cover: Ukrainian soldiers stand in front of a huge Soviet-era arch in Kiev on December 2, 2018, symbolizing Ukraine and Russia's friendship and union, where an unknown artist added a crack following recent tensions. /VCG Photo)