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STORY: Camouflaged among the leaves, a Ukrainian artillery unit in the embattled Donbas region stands ready to join the fight against invading Russian forces.

The soldiers here offered Reuters a peek into their concealed bunkers, and a view of the relative calm around the campfire as they await orders.

Volodymyr Dumanskyi is a member of the unit.

"We are ready for any development of events: we are ready to attack, ready to defend, ready to retreat - if necessary. We are ready for anything."

The Donbas is the focal point of current hostilities in Ukraine. Russia is waging a major offensive in Luhansk, one of two provinces in Donbas.

Russian forces attacked Ukraine almost three months ago in what President Vladimir Putin called a "special military operation" to remove what it called dangerous nationalists from power.

Ukraine and western governments dismissed this justification as false and accuse Russia of launching an unprovoked invasion.

A spokesperson from the Russian defense ministry on Sunday said artillery and airstrikes pummeled Ukrainian targets. Reuters was unable to independently verify those reports.

Russian forces have been trying to seize the eastern region since mid-April after an initial push to capture Kyiv failed in the face of Ukrainian resistance.

As the fighting moves east, Kyiv appears to have hardened its stance on negotiations.

Ukraine on Sunday ruled out a ceasefire or any territorial concessions to Russia.

The chief of staff to Ukrainian President Volodymyr Zelenskiy tweeted, "the war must end with the complete restoration of Ukraine's territorial integrity and sovereignty."

Zelenskiy met with Polish leader Andrzej Duda on Sunday. Duda spoke to the Ukrainian parliament, saying Warsaw backed Kyiv in demanding a full Russian withdrawal.

Here in the forest, among the artillery shells and javelin anti-tank rockets, this team of artillerymen find a moment to laugh before the fighting resumes.