More than a dozen advanced Russian fighter jets have arrived at a base in the Ukrainian region of Crimea, which Moscow occupied and annexed in 2014.
The Reuters news agency cited witnesses who saw the Su-27 and Su-30 fighters arriving at the Belbek air base on December 22.
The move comes amid high tensions between Ukraine and Russia across the region, which were raised on November 25 when Russia seized three Ukrainian Navy vessels and their crews near the Kerch Strait, which connects the Black Sea and the Sea of Azov.
Russian Foreign Minister Sergei Lavrov has said Ukraine was preparing a "provocation" in the area before the end of the year.
Kyiv, for its part, has charged that Russia was building up military forces along the two countries' mutual border. President Petro Poroshenko has said Russia has moved "more than 80,000 troops, 1,400 artillery and multiple rocket launch systems, 900 tanks, 2,300 armored combat vehicles" and hundreds of aircraft to the area.