(NewsNation) — In a first for the Ukrainian military, troops have launched a long-awaited counter-offensive to retake territory in the south as Russian forces shelled residential areas of the Black Sea port city of Mykolaiv.

Ukrainian President  Voldymyr Zelenskyy is now calling on Russian troops to flee across the border as Ukrainians make gains to retake areas now held by Russian troops. 

Moscow acknowledged a new offensive had been launched, but said it had failed and the Ukrainians had suffered significant casualties.

A former deputy who switched allegiance from Ukraine to Russia was shot dead in the city of Kherson overnight, and more strikes are continuing in the southern part of Ukraine.

But a Ukrainian barrage of rockets left the Russian-occupied town of Nova Kakhovka without water or power, officials at the Russian-appointed local authority told RIA news agency.

UK Intelligence suggest Tuesday that Russia has been trying to reinforce troops along the Dnieper River for the past month.

Ukraine’s new military attack came after several weeks of relative stalemate in a war that has killed thousands, displaced millions, destroyed cities and caused a global energy and food crisis amid unprecedented economic sanctions.

Russian shelling of Mykolaiv killed at least two people, injured some 24 others and wiped out homes, according to city officials and witnesses.

The conflict, the biggest attack on a European state since 1945, had largely settled into a war of attrition, mainly in the south and east, marked by artillery bombardments and air strikes.

Russia captured swaths of Ukraine’s south near the Black Sea coast in its early phase.

Ukraine’s southern command said its troops had started offensive actions in several directions in the south, including in the Kherson region which lies north of the Russian-annexed Crimean peninsula.

Ukraine had struck more than 10 sites in the past week and “unquestionably weakened the enemy”, according to a spokeswoman who declined to give details of the counter-offensive, saying Russian forces in the south remained “quite powerful”.

Ukraine has been using sophisticated Western-supplied weapons to hit Russian ammunition dumps and wreak havoc with supply lines.

Russia’s defense ministry said Ukrainian troops had attempted an offensive in the southern Mykolaiv and Kherson regions but sustained significant casualties, RIA news agency reported

“(The) enemy’s offensive attempt failed miserably,” it said.

Reuters could not independently verify the battlefield reports.

In Mykolaiv, where Russian shelling hit residential areas, a Reuters correspondent said a strike hit a family home directly next to a school, killing one woman.

The owner of the property, Olexandr Shulga, told Reuters he had lived there his entire life and that his wife – who was in a different room when the missile struck – died when she was buried in debris.

“It hit and the shockwave came. It destroyed everything,” he said.

A shipbuilding center and port on the Southern Bug river just off the Black Sea, Mykolaiv has suffered heavy Russian bombardments throughout the war but remained in Ukrainian hands.

Russia says it is waging a “special military operation” in Ukraine to rid it of nationalists and protect Russian-speaking communities. Ukraine and the West describe Russia’s actions as an unprovoked war of aggression.

Reuters contributed to this report.