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, the capital, 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 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 Moscow appears to hope that local support, logistics and the terrain in the region favor its larger, better-armed military, potentially allowing Russia to finally turn the tide in its 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.

Ukrainian President Volodymyr Zelenskyy said that while experts try to determine what the substance might be, “The world must react now.” Evidence of “inhuman cruelty” toward women and children in Bucha and other suburbs of Kyiv continued to surface, he added, including of alleged rapes.

“Not all serial rapists reach the cruelty of Russian soldiers,” Zelenskyy said.

The claims 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 leveled 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.

Zelenskyy adviser Mykhailo Podolyak acknowledged the challenges Ukrainian troops face in Mariupol. He said via Twitter that they remain blocked and are having issues with supplies, while Ukraine’s president and generals “do everything possible (and impossible) to find a solution.”

“For more than 1.5 months our defenders protect the city from (Russian) troops, which are 10+ times larger,” Podolyak tweeted. “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.” He was even blunter later Tuesday, repeating the term and saying: “It’s become clearer and clearer that Putin is just trying to wipe out the idea of even being a Ukrainian.”

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.”

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 Washington, a senior U.S. defense official said the Biden administration was preparing yet another package of military aid for Ukraine possibly totaling $750 million to be announced in the coming days. The official spoke on condition of anonymity to discuss plans not yet publicly announced. Delivery is due to be completed this week of $800 million in military assistance approved by Biden a month ago.

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 killing thousands. The war has driven more than 10 million Ukrainians from their homes — including nearly two-thirds of the country’s children.

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

More than 720 people were killed in Kyiv suburbs that had been occupied by Russian troops and over 200 were considered missing, the Interior Ministry said early Wednesday.

In Bucha alone, Mayor Anatoliy Fedoruk said 403 bodies had been found and the toll could rise as minesweepers comb the area.

Ukraine’s prosecutor-general’s office said Tuesday it was also looking into events in the Brovary district, which lies to the northeast.

It said the bodies of six civilians were found with gunshot wounds in a basement in the village of Shevchenkove and 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 journalists saw dozens of bodies in and around the town, some of whom had their hands bound and appeared to have been shot at close range.

Speaking at the Vostochny space launch facility in Russia’s far east, in his first known foray outside Moscow since the war began, Putin also said the West would fail to isolate Russia and its economy has withstood a “blitz” of sanctions.

Addressing the pace of the campaign, he said Moscow was proceeding “calmly and rhythmically” to “achieve the planned goals while minimizing the losses.”

The Russian 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.

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Karmanau reported from Lviv, Ukraine. Associated Press writer Robert Burns in Washington, and AP journalists around the world contributed to this report.

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Follow the AP’s coverage of the war at https://apnews.com/hub/russia-ukraine