KYIV, Ukraine (AP) — President Vladimir Putin vowed Tuesday that Russia’s bloody offensive in Ukraine would continue until its goals are fulfilled, and insisted the campaign was going as planned, despite a major withdrawal in the face of stiff Ukrainian opposition and significant losses.

Russian troops, thwarted in their push toward Ukraine’s capital, are now focusing on the eastern Donbas region, where Ukraine said Tuesday 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.

Russia invaded on February 24, with the goal, according to Western officials, of taking Kyiv, toppling the government and installing a Moscow-friendly one. In the six weeks since, Russia’s ground campaign stalled, its forces suffered losses that may number in the thousands and it stands accused of killing civilians and other atrocities.

Putin insisted Tuesday that his military action aimed to protect people in areas in eastern Ukraine controlled by Moscow-backed rebels and to “ensure Russia’s own security.”

He said Russia “had no other choice” but to launch what he calls a “special military operation,” and vowed it would “continue until its full completion and the fulfillment of the tasks that have been set.”

For now, Putin’s forces are gearing up for a major offensive in the Donbas which has been torn by fighting between Russian-allied separatists and Ukrainian forces since 2014, and where Russia has recognised the separatists’ claims of independence. Military strategists say Russian leaders appear to hope local support, logistics and terrain in the region favor Russia’s larger and better-armed military, potentially allowing its troops to finally turn the tide in their favor.

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

It came after a Russia-allied separatist official appeared to urge the use of chemical weapons, telling Russian state TV on Monday that separatist forces should seize the plant by first blocking all the exits. “And then we’ll use chemical troops to smoke them out of there,” the official, Eduard Basurin, said. He denied Tuesday that separatist forces had used chemical weapons in Mariupol.

Ukraine’s Deputy Defense Minister Hanna Maliar said officials were investigating, and it was possible phosphorus munitions — which cause horrendous burns but are not classed as chemical weapons — had been used in Mariupol.

That city has been razed in six weeks of pummelling by Russian troops that the mayor said Monday has left more than 10,000 civilians dead, their corpses “carpeted through the streets.” Mayor Vadym Boychenko said the death toll in Mariupol alone could surpass 20,000 and gave new details of allegations by Ukrainian officials that Russian forces have brought mobile cremation equipment to dispose of the corpses.

In the face of stiff resistance by Ukrainian forces bolstered by Western weapons, Russian forces have increasingly relied on bombarding cities, flattening many urban areas and leaving thousands of people dead. The war has also driven more than 10 million Ukrainians from their homes — including nearly two-thirds of all children.

Moscow’s retreat from cities and towns around the capital, Kyiv, led to the discovery of large numbers of apparently massacred civilians, prompting widespread condemnation and accusations that Russia is committing war crimes in Ukraine.

Putin falsely claimed Tuesday that Ukraine’s accusation that hundreds of civilians were killed by Russian troops in the town of Bucha were “fake.” Associated Press reporters saw dozens of bodies in and around the town, some with hands bound who appeared to have been shot at close range.