Russia launched air strikes deeper into Ukraine and called on foreign “volunteers” to join its war on Friday as Moscow stepped up unfounded allegations over chemical weapons threats.

With the war in its third week, Russian jets hit the central Ukrainian city of Dnipro while rockets pounded Lutsk in the west, in a sign President Vladimir Putin is widening the scope of an assault that has targeted the country’s largest population centres.

Chairing a session of his security council via video link, Russia’s president ordered his army to deploy foreign “volunteers” to the conflict zone to “help” residents of Ukraine. Sergei Shoigu, Russia’s defense minister, told Putin that 16,000 from the Middle East were prepared to fight, many with experience of battling Isis.

Putin’s initiative came as Russia expanded its aerial assault on Ukraine’s cities while shifting parts of an armoured column north of Kyiv into better positions to resume its stalled offensive on the capital.

A video posted on social media showed a huge explosion and fireball on the outskirts of Dnipro early on Friday morning. Emergency services said the strikes were near a kindergarten and an apartment building and that one person died in the attack.

The Russian air assault followed talks between the Russian and Ukrainian foreign ministers on Thursday that failed to make progress towards ending the war, or even establishing a temporary ceasefire.

Pressing a narrative that has alarmed the west, Moscow requested the UN Security Council to meet on Friday to discuss allegations that the US had funded unconventional weapons research in Ukraine. The White House has said the allegations are “preposterous” and warned, along with the UK, that the false claims have been manufactured by Moscow to potentially justify its own use of chemical or biological weapons.

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Over the course of 16 days, Russia has so far failed to capture any large city and is far from achieving its goal of disarming Ukraine’s military and ousting its government.

But heavy shelling of Ukrainian cities such as Kharkiv and Mariupol has exacted a huge human toll and forced more than 2mn people to flee the country in search of safety. After a summit of its leaders in Versailles, the EU promised on Friday to offer temporary protection to “all war refugees from Ukraine”.

US vice-president Kamala Harris backed calls for a war crimes investigation into Putin’s invasion, citing civilian “atrocities” caused by indiscriminate bombing. “Absolutely there should be an investigation, and we should all be watching,” she said on a visit to Poland.

Russia’s traditional allies, oligarchs and friendly ex-politicians have made various attempts to edge the Russian president towards a settlement, including the former German chancellor Gerhard Schröder, who met Putin on Thursday, according to the German news agency DPA.

Schröder’s wife Kim So-yeon posted a photo of herself with her hands clasped together as if in prayer, next to a window, with the night-time skyline of Red Square illuminated behind her.

Russia’s economy is reeling from sanctions and Europe and the US have continued to tighten curbs on its ability to trade, including exploring whether to deprive Russia of “most favoured nation” status at the World Trade Organization, which would increase tariffs on its exports.

On Thursday, Putin vowed to seize the Russian assets of international businesses that have suspended or closed their operations in the country — companies ranging from McDonald’s to Ikea — using “legal solutions”.

Ukrainian officials described the situation faced by civilians trapped in Mariupol, which has been subjected to a remorseless bombardment, as particularly dire.

Residents have no heat or phone service and have been confined for days in freezing shelters, with food and water supplies running out. Bodies have been buried in mass graves. Deputy prime minister Iryna Vereshchuk said more than 1,300 people had so far died in the 10-day siege of the city.

“They have a clear order to hold Mariupol hostage, to mock it, to constantly bomb and shell it,” Ukrainian president Volodymyr Zelensky said in a video address. He accused the Russians of staging a tank attack in an area where there was supposed to be a humanitarian corridor out of Mariupol.

In an update on Friday morning, Ukraine’s military said it had been “repelling and holding back” Russian forces making no significant advances from its frontline position.

Britain’s defence ministry noted that persistent logistical issues were still hampering Russia’s military and that Moscow was likely “to reset and reposture its forces for renewed offensive activity in coming days”.

Western defence advisors said they believed the massive column north of Kyiv had split in two parts, making it harder to attack. One column, thought to be made up of the elite 1st Tanks Guard army, had now dispersed into the woods and countryside around the capital in preparation for an assault, they said.

The Russian defence ministry announced that Russian forces had taken control of Volnovakha, a besieged town in south-eastern Ukraine. The ministry also announced that its forces had launched rocket attacks that had disabled military airfields in Lutsk and Ivano-Frankivsk in western Ukraine and were advancing on Mariupol.

As Russia’s offensive has faltered around Kyiv and Kharkiv, Ukraine’s biggest cities, Moscow’s repeated claims over chemical and biological weapons programmes in Ukraine have raised western concerns Putin may resort to the use of unconventional weapons.

In his late-night address, Zelensky described Moscow’s claims as a smokescreen to justify it deploying ever more inhuman weapons, asking Putin: “What else have you prepared for us?”

“Allegedly, we are preparing a chemical attack,” he said in the address. “This makes me really worried, because we’ve been repeatedly convinced: if you want to know Russia’s plans, look at what Russia accuses others of [doing].”

A senior US defence official said there were no American biological weapons labs in Ukraine “or anywhere else in the world”, adding that Washington spent $200mn in Ukraine to “eliminate the remnants” of an illegal Soviet-era biological weapons programme.

As the west continued to tighten sanctions against Russia, the US Senate on Thursday evening approved a $13.6bn spending package that includes $6.5bn dedicated to defence. The bill will now pass to US president Joe Biden to sign into law.

Adjusted for inflation, the support exceeds the landmark $400mn package the US provided Greece and Turkey in 1947. That move marked the start of US cold war policy in Europe and, two years later, the foundation of Nato.

Additional reporting by Erika Solomon in Berlin, Aime Williams in Washington, John Paul Rathbone in London and Victor Mallet in Paris