Russia vowed to continue its bloody offensive in Ukraine as the war neared its seventh week Wednesday, as President Vladimir Putin insisted the campaign was going as planned despite a major withdrawal and significant losses.

Thwarted in their push toward the capital, Kyiv, Russian troops focused on the eastern region of Donbas, where Ukraine said it was investigating a claim that a poisonous substance had been dropped on its troops. It was not clear what the substance might be, but Western officials warned that any use of chemical weapons by Russia would be a serious escalation of the already devastating war.

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Russia invaded on Feb. 24 with the goal, according to Western officials, of taking Kyiv, toppling the government and installing a Moscow-friendly regime. In the six weeks since, the ground advance stalled and Russian forces lost potentially thousands of fighters and were accused of killing civilians and other atrocities.

Putin said Tuesday that Moscow “had no other choice” and that the invasion aimed to protect people in parts of eastern Ukraine and to “ensure Russia’s own security.” He vowed it would “continue until its full completion and the fulfilment of the tasks that have been set.”

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For now, Putin’s forces are gearing up for a major offensive in the Donbas, where Russian-allied separatists and Ukrainian forces have been fighting since 2014, and where Russia has recognized the separatists’ claims of independence. Military strategists say Moscow believes local support, logistics and the terrain in the region favour its larger, better-armed military, potentially allowing Russia to finally turn the tide in its favour.

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In Mariupol, a strategic port city in the Donbas, a Ukrainian regiment defending a steel mill alleged that a drone dropped a poisonous substance on the city. The assertion by the Azov Regiment, a far-right group now part of the Ukrainian military, could not be independently verified. The regiment indicated there were no serious injuries.