KYIV, Ukraine (AP) -- 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 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, Russia's ground advance stalled, its forces lost potentially thousands of fighters and the military was accused of killing civilians and other atrocities.
Putin insisted Tuesday that his invasion aimed to protect people in parts of 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 recognized the separatists' claims of independence. Military strategists say Russian leaders appear to hope that local support, logistics and the terrain in the region favor Russia's larger, 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 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.
Much of the city has been razed in weeks of pummeling by Russian troops. The mayor said Monday that the siege has left more than 10,000 civilians dead, their bodies "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.
Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, acknowledged the challenges Ukrainian troops face in Mariupol. He said on Twitter that they remain blocked and are having issues with supplies, while Zelenskyy and Ukrainian generals "do everything possible (and impossible) to find a solution and help our guys."
"For more than 1.5 months our defenders protect the city from (Russian) troops, which are 10+ times larger," Podolyak said in a tweet. "They're fighting under the bombs for each meter of the city. They make (Russia) pay an exorbitant price."
British Foreign Secretary Liz Truss said the use of chemical weapons "would be a callous escalation in this conflict," while Australian Foreign Minister Marise Payne said it would be a "wholesale breach of international law."
U.S. President Joe Biden for the first time referred to Russia's invasion as a "genocide," linking the war to rising prices. "Your family budget, your ability to fill up your tank, none of it should hinge on whether a dictator declares war and commits genocide a half a world away," he said.
Pentagon spokesman John Kirby said in a statement that the U.S. could not confirm the drone report. But he noted the administration's persistent concerns "about Russia's potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine."
Britain, meanwhile, has warned that Russia may resort to phosphorus bombs, which are banned in civilian areas under international law, in Mariupol.
Most armies use phosphorus munitions to illuminate targets or to produce smoke screens. Deliberately firing them into an enclosed space to expose people to fumes could breach the Chemical Weapons Convention, said Marc-Michael Blum, a former laboratory head at the Netherlands-based Organization for the Prohibition of Chemical Weapons.
"Once you start using the properties of white phosphorus, toxic properties, specifically and deliberately, then it becomes banned," he said.
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.
Reports have primarily focused on the northwestern suburbs such as Bucha, where the mayor said 403 bodies have been found. Mayor Anatoliy Fedoruk feared the toll would rise as minesweepers comb through the area.
Ukraine's prosecutor-general's office said Tuesday that it was also looking into events in the Brovary district, which lies to the northeast.
The prosecutor's office said the bodies of six civilians had been found with gunshot wounds in a basement in the village of Shevchenkove and that Russian forces were believed to be responsible.
Prosecutors are also investigating allegations that Russian forces fired on a convoy of civilians trying to leave by car from the village of Peremoha in the Brovary district, killing four people, including a 13-year-old boy. In another attack near Bucha, five people were killed, including two children, when a car was fired upon, prosecutors said.
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.
The Russian leader spoke at the Vostochny space launch facility in the country's far east, during his first known foray outside Moscow since the war began. He also said that foreign powers wouldn't succeed in isolating Russia.
He said that Russia's economy and financial system withstood the blow from what he called the Western sanctions "blitz" and claimed they would backfire by driving up prices for essentials such as fertilizer, leading to food shortages and increase migration flows to the West.
Addressing the pace of the campaign, Putin said Russia was proceeding "calmly and rhythmically" because it wanted to "achieve the planned goals while minimizing the losses."
While building up forces in the east, Russia continued to strike targets across Ukraine in a bid to wear down the country's defenses. Russia's defense ministry said Tuesday that it used used air- and sea-launched missiles to destroy an ammunition depot and airplane hangar at Starokostiantyniv in the western Khmelnytskyi region and an ammunition depot near Kyiv.