"The situation is tense in all areas of the front line. The east is the epicentre," Ukrainian Deputy Defence Minister, Ganna Malyar, wrote on Telegram.

"The enemy continues to concentrate its main efforts on the Lyman, Bakhmut, Avdiivka and Maryinka directions", she added, referring to eastern cities where fighting has been raging for months.

The deputy minister gave few details on the situation in southern Ukraine.

Moscow says fighting has intensified since Thursday in the Zaporizhzhia region, particularly around the small town of Orikhiv.

Orikhiv is on the road towards the larger city of Tokmak, around 40 kilometres (25 miles) to the south in Russian-occupied territory.

"The enemy is conducting defensive actions in the Zaporizhzhia sector," Malyar said.

Ukraine has said for months that it is preparing for a large-scale offensive, aimed at retaking territory occupied by Russia in the south and east.

The resumption of fighting in the south has been seen by many observers as a sign that the counter-offensive is already happening, or is about to start.

A Ukrainian military success in Zaporizhzhia region would enable its forces to break through the land bridge that connects Russia with the Crimean peninsula it annexed from Ukraine. This would be a major reverse for Moscow.

But the Russian army in recent months has strengthened its front lines in the region, digging kilometres of trenches and fortifying its defences.