Firefighters work to extinguish a fire after a Russian attack destroyed a building in Kharkiv on Tuesday. AP

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

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A highway through the rolling plains of eastern Ukraine looks set to play an important strategic role in the anticipated Russian offensive in the Donbas.

Connecting the regions of Kharkiv and Donetsk, the two-lane thoroughfare is one of the key routes in the northeast of the country.

Russia's President Vladimir Putin visits the Vostochny cosmodrome in Amur region, on Tuesday. AFP

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.

The road runs like an arrow to the centre of the Ukrainian-controlled Donbas, caught between pro-Russian statelets of Donetsk to the south and Lugansk to the east, and the advance of Russian forces from the north.

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 fulfillment of the tasks that have been set.”

This satellite image shows a convoy of armoured vehicles and trucks on a highway near Kharkiv, Ukraine. AFP

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 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 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.

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

Firefighters try to extinguish a fire after a missile hits a building on the outskirts of Kharkiv on Tuesday. AFP

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.

Ukrainian 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, which has been pummeled by weeks of Russian assaults.

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