Ukraine’s Commander-in-Chief of the Armed Forces told his US counterpart that Russia was concentrating its main efforts in the northern part of the Luhansk region, in the country's east.

Valerii Zaluzhnyi briefed General Mark A. Milley, the US chairman of the Joint Chiefs of Staff, on the latest battlefield updates in Ukraine, he said in a Telegram post on Sunday.

He also said Russia was using “artillery en masse and unfortunately has a tenfold fire superiority" but that Ukraine was holding its positions there.

Remember: Luhansk region in eastern Ukraine is at the epicenter of the conflict. Much of the area is already under Russian control.

In the east: Zaluzhnyi described the situation in Luhansk as “complicated, in particular, in the city of Severodonetsk. Up to seven battalion tactical groups were deployed by the enemy there. Despite the heavy fire, we managed to stop the enemy."

In the north: Zaluzhnyi said in northern and northeastern Ukraine, Russian forces “resumed shelling the residential areas of Kharkiv, which requires us to take adequate action.”

He also said Russian forces had targeted the Chernihiv and Sumy regions using artillery and mortar fire, adding “we're firing back.”

The front line: Zaluzhnyi said there is a 2,450 kilometer (1,522 miles) frontline, of which 1,105 kilometers (686 miles) is in “active hostilities."

He asked General Miley “to help us get more 155 mm caliber artillery systems in the shortest possible time."