Ukrainian President Petro Poroshenko has said that Russia is amassing ground forces and weaponry along the border between the two countries.
Speaking at a military event in Kyiv on December 1, Poroshenko said Russia has deployed "more than 80,000 troops, 1,400 artillery and multiple-rocket-launch systems, 900 tanks, 2,300 armored combat vehicles, 500 airplanes, and 300 helicopters" near the border.
He also said Russia had 80 ships and eight submarines in the Aegean Sea, the Black Sea, and the Sea of Azov.
The numbers could not be independently verified.
The alleged buildup comes in the wake of a November 25 incident in which Russia seized three Ukrainian naval vessels and detained 24 Ukrainian sailors near the Kerch Strait.
"The Kremlin is further testing the strength of the global order," Poroshenko said, adding that Moscow wants to see if the international community will allow Russia to claim the Black Sea and the Sea of Azov as its territorial waters.
The European Union and the United States have urged Russia to release the Ukrainian vessels and servicemen immediately.
In 2014, Russia annexed the Ukrainian region of Crimea.
Moscow also provides military, political, and economic support to separatist formations in eastern Ukraine.
The conflict in the eastern part of Ukraine has claimed more than 10,300 lives since April 2014.