Russian forces were launching an all-out assault to encircle Ukrainian troops in twin cities straddling a river in eastern Ukraine on Tuesday, a battle which could determine the success or failure of Moscow’s main campaign in the east.

Exactly three months after Russian President Vladimir Putin invaded Ukraine, authorities in its second-largest city Kharkiv re-opened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment.

The reopening was evidence of Ukraine’s biggest military success over the past few weeks: pushing Russian forces largely out of artillery range of Kharkiv, as they did from the capital Kyiv in March.

But the decisive battles of the war’s latest phase are still raging further south, where Moscow is attempting to seize the Donbas region of two eastern provinces, Donetsk and Luhansk, and trap Ukrainian forces in a pocket on the main eastern front.

The easternmost part of the Ukrainian-held Donbas pocket, the city of Sievierodonetsk on the east bank of the Siverskiy Donets river and its twin Lysychansk on the west bank, have become the pivotal battlefield there, with Russian forces advancing from three directions to encircle them.

“The enemy has focused its efforts on carrying out an offensive in order to encircle Lysychansk and Sievierodonetsk,” said Serhiy Gaidai, governor of Luhansk province, where the two cities are among the last territory still held by Ukraine.

“The intensity of fire on Sievierodonetsk has increased by multiple times, they are simply destroying the city,” he said on TV, adding there were about 15,000 people in the city.

Reuters journalists in the Donbas, who reached Bakhmut further west, heard and saw intense shelling on the highway towards Lysychansk on Monday.

Ukrainian armoured vehicles, tanks and rocket launchers were moving towards the front lines, with and buses carrying soldiers.

Further west in Slovyansk, one of the biggest Donbas cities still in Ukrainian hands, air raid sirens wailed on Tuesday morning but streets were still busy, with a market full, children riding bikes and a street musician playing violin by a supermarket.

Two empty buses were driving towards the frontline town of Lyman to evacuate civilians from heavy shelling there, escorted by the police and a military car.