Ukrainian exercisation is building fortifications in the occupied territories in the Kursk region, Russia, local media report.
Ukrainian soldiers have managed to take control of some 450 square kilometers inside Russia after a surprise operation that began five days ago.
According to reports, the Ukrainian military is building fortifications and undermining the area, with the aim of keeping the territory occupied for longer. Meanwhile, other ukrainian troops are still entering Russian territory.
There was also a reported military activity by Ukrainian special forces in several Russian towns overnight.
It is believed that there are about 10,000 Ukrainian soldiers inside the country, backed by heavy combat vehicles such as tanks, air-to-air systems and other weapons.
It is widely thought that the invasion inside Russia was undertaken to ask the Kremlin in future to swap territory, but also to force the Russian military to move from Ukraine to Russia.
Although information is scarce, and mostly only from Russian reports, it is suspected that about 400 Russian soldiers have been killed so far, more than 200 have been captured, and losses have also been found among the Ukrainian army.