Three months into the Russian invasion of Ukraine, Moscow 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.

Ukrainian authorities on the other hand are expected to open the underground metro in the second-largest city Kharkiv, where thousands of civilians had sought shelter for months under relentless bombardment.

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.

Regional governor Serhiy Haidai assured that Severodonetsk in the eastern Donbas region is still under the control of Ukraine despite Russian forces bombarding the city.

The governor wrote on Telegram that "the Russians concentrated almost all their forces, namely 25 battalion tactical groups to take the city".

Russian forces are shelling the centre of the city "chaotically and uninterruptedly", while battles are taking place in the surrounding villages, he said.

Key events in the conflict till date
On February 24, Russia invaded Ukraine from three fronts, the biggest such assault on a European state since World War Two, claiming to "denazify" Ukraine in a a "special military operation"

President Volodymyr Zelenskiy refused to backdown saying "Russia has embarked on a path of evil, but Ukraine is defending itself."

On March 2, Russian troops reach the centre of the Black Sea port of Kherson, the first large urban centre to be captured.

One million people have fled Ukraine, reports the U.N. refugee agency (UNHCR). By March 8 the number doubled to a staggering Two million.

On March 16, Russia bombs a Mariupol theatre where hundreds of civilians are sheltering.

Moscow on March 25, signals it is scaling back its ambitions and will focus on making gains in the east, and Ukrainian forces go on the offensive to recapture towns outside Kyiv.

By March 30, UNHCr reports that more than 4 million people have fled Ukraine.

The discovery of mass graves in Bucha, by Ukraine on April 3 and 4 prompts the country to accuse Russia of war crimes

On April 8, Russia allegedly fires a at a train station in Kramatorsk, killing at least 52 people trying to flee the looming eastern offensive.

Russia's lead warship in the Black Sea, the Moskva, sinks on April 14th after what Ukraine says was a missile strike. Russia blamed it on an ammunition explosion.

On April 21, Putin declares Mariupol "liberated" after nearly two months of siege, with hundreds of defenders hold out inside the city's huge Azovstal steelworks.

On April 28, Russia fires two missiles into Kyiv during a visit by U.N. Secretary-General Antonio Guterres.

On May 1, United Nations evacuates about 100 Ukrainian civilians from Mariupol's ruined Azovstal steelworks, in a "safe passage operation".

On May 7, The regional governor reports of 60 people dead after a bomb strikes a village school in Bilohorivka, eastern Ukraine.

By May 12, More than 6 million people have fled Ukraine, reports the UNHCR.

On May 18, Finland and Sweden formally apply to join the NATO alliance, a move that would bring about the expansion of the Western military alliance that Putin aimed to prevent.

On May 20, Ukrainian fighters holding out at Mariupol's Azovstal steelworks surrender to Russian forces over a period of several days.

Russia's three-month long invasion, the biggest attack on a European state since 1945, has seen more than 6.5 million people flee abroad, turned entire cities into rubble and brought down severe economic sanctions on Moscow.

At a cemetery outside Mariupol, reports Associated Press, treading through long rows of fresh graves and makeshift wooden crosses, Natalya Voloshina, who lost her 28-year-old son in the fight for the city, said many of Mariupol's dead had no one left to honour their memory.

"Who will bury them? Who will put up a plaque?" she asked. "They have no family."
-Reuters/AP/BBC