In his nightly address Zelensky said his forces were inflicting heavy casualties on the Russians and that "In many respects, the fate of the Donbas is being decided there.
He said: “This is a very fierce battle, very hard, perhaps one of the most difficult of the entire war.
"In many ways, it is there that the fate of our Donbas is being decide”
The region has been partly controlled by Russia-backed separatists for years, making supply lines shorter and allowing Moscow to tap separatist forces to back its offensive.
But Russia also faces Ukraine’s most battle-hardened troops, who have been fighting the separatists for eight years there.
Severodonetsk became the administrative capital of the region after the city of Luhansk was taken by separatists in 2014. Both it and Lysychansk are wedged between Russian forces to the east, north and south, and are among a few cities and towns in Luhansk still holding out.
The Kremlin has claimed its forces hold nearly all of the Luhansk region, and about half of the Donetsk region that rounds out the Donbas.
The UK’s Ministry of Defence (MoD) said in an update that Russia were seeking to gain momentum in the conflict and upping their efforts to secure the territory.
An update from MoD read: “Fighting continues in the Sieverodonetsk pocket but, in the last 48 hours, Russia’s Eastern Group of Forces (EGF) have also likely increased their efforts to advance to the south of Izium.
"Russia’s progress on the Izium axis had remained stalled since April, after Ukrainian forces made good use of the terrain to slow Russia’s advance.”
It added: “Russia has likely attempted to reconstitute EGF after they suffered very heavy casualties in the failed advance on Kyiv, but its units likely remain understrength.
"Russia likely seeks to regain momentum in this area in order to put further pressure on Sieverodonetsk, and to give it the option of advancing deeper into the Donetsk Oblast.”
Russia and Turkey have voiced support for the creation of a safe maritime corridor in the Black Sea so Ukraine can export grain to global markets amid an escalating world food crisis.
But Russia demanded that the Black Sea be demined and Turkey said allowing the Ukraine exports should be accompanied by easing of western sanctions against Russia.
Ukraine is one of the world’s largest exporters of wheat, corn and sunflower oil, but the war and a Russian blockade of its ports have halted much of that flow, endangering food supplies to many developing countries. Many of these ports are now also heavily mined.
Both Russia and Turkey would benefit from the export plan. The same corridor would also allow Russia to safely export its food and fertiliser. Turkey says it would facilitate and protect the transport of the grain in the Black Sea — a move cutting out other possible shippers.
Mr Zelensky insisted on Ukraine’s need to defeat Russia on the battlefield but also said he was still open to peace talks with Russian president Vladimir Putin.