Russia vowed to continue its bloody offensive in Ukraine as the war neared its seventh week, 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.

Russia invaded on February 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.

Mr Putin said 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”.

For now, Mr 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 recognised 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.

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.

A view inside the Mariupol theatre damaged during fighting in Mariupol (Alexei Alexandrov/AP)
A view inside the Mariupol theatre damaged during fighting in Mariupol (Alexei Alexandrov/AP)

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.

Ukrainian President Volodymyr Zelensky said that while experts try to determine what the substance might be: “The world must react now.”

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.

(PA Graphics)

He denied that separatist forces had used chemical weapons in Mariupol.

Ukrainian deputy defence 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, which has been pummelled by weeks of Russian assaults.

Western leaders warned that if chemical weapons are found to have been used, it would amount to a grievous breach of international law.

US President Joe Biden for the first time referred to Russia’s invasion as a “genocide” and said “Putin is just trying to wipe out the idea of even being a Ukrainian.”

The Pentagon said it could not confirm the drone report but reiterated U.S. concerns about Russia using chemical agents.

Russian President Vladimir Putin (Evgeny Blyatov/AP)
Russian President Vladimir Putin (Evgeny Blyatov/AP)

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 US defence official said the Biden administration was preparing yet another package of military aid for Ukraine to be announced in the coming days, possibly totalling 750 million US dollars.

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 US dollars in military assistance approved by Mr 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.

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Refugees walk after fleeing the war from neighbouring Ukraine at the border crossing in Medyka, southeastern Poland (Sergei Grits/AP)

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.

Mr Zelensky said evidence of “inhuman cruelty” toward women and children in Bucha and other suburbs of Kyiv continued to surface, including alleged rapes.

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

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 on 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 saidit 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.

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Ukrainian President Volodymyr Zelenskyy speaks from Kyiv (Ukrainian Presidential Office/AP)

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.

Mr Putin falsely claimed that Ukraine’s accusation that hundreds of civilians were killed by Russian troops in the town of Bucha were “fake”.

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, Mr Putin also said the West would fail to isolate Russia and its economy has withstood sanctions.

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