President Joe Biden and western allies have pledged new sanctions and humanitarian aid in response to Vladimir Putin’s assault on Ukraine, but their offers fell short of Ukrainian President Volodymyr Zelensky’s pleas.

Mr Biden also announced the US would welcome up to 100,000 Ukrainian refugees — though he said many probably prefer to stay closer to home — and provide an additional 1 billion dollars (£760 million) in food, medicine, water and other supplies.

The western leaders spent Thursday crafting next steps to counter Russia’s month-old invasion — and huddling over how they might respond should Mr Putin deploy chemical, biological or even a nuclear weapon.

They met in a trio of emergency summits that had them shuttling across Brussels for back-to-back-to-back meetings of Nato, G7 and the 27-member European Council.

Mr Biden, in an early evening news conference after the meetings, warned that a chemical attack by Russia “would trigger a response in kind”.

“You’re asking whether Nato would cross. We’d make that decision at the time,” Mr Biden said.

However, a White House official said later that did not imply any shift in the US position against direct military action in Ukraine. Mr Biden and Nato allies have stressed that the US and Nato would not put troops on the ground in Ukraine.

The official was not authorised to comment publicly by name and spoke only on condition of anonymity.

Mr Zelensky, while thankful for the newly promised help, made clear to the western allies he needed far more than they are currently willing to give.

“One percent of all your planes, one percent of all your tanks,” Mr Zelensky asked members of the Nato alliance. “We can’t just buy those. When we will have all this, it will give us, just like you, 100% security.”

Mr Biden said more aid was on its way. But the western leaders were treading carefully so as not to further escalate the conflict beyond the borders of Ukraine.

“Nato has made a choice to support Ukraine in this war without going to war with Russia,” said French President Emmanuel Macron. “Therefore we have decided to intensify our ongoing work to prevent any escalation and to get organised in case there is an escalation.”

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Ukrainian President Volodymyr Zelensky speaks to the leaders of the European Council during their summit in Brussels from Kyiv, Ukraine (Ukrainian Presidential Press Office via AP)

Poland and other eastern flank Nato countries are seeking clarity on how the US and European nations can assist in dealing with their growing concerns about Russian aggression as well as the refugee crisis.

More than 3.5 million refugees have fled Ukraine in recent weeks, including more than 2 million to Poland.

Mr Biden is to visit Rzeszow, Poland, on Friday, where energy and refugee issues are expected to be at the centre of talks with President Andrzej Duda.

He will get a briefing on humanitarian aid efforts to assist fleeing refugees and he will meet with US troops from the 82nd Airborne Division who have been deployed in recent weeks to bolster Nato’s eastern flank.

Billions of dollars of military hardware have already been provided to Ukraine. A US official, who requested anonymity to discuss internal deliberations, said western nations were discussing the possibility of providing anti-ship weapons amid concerns that Russia will launch amphibious assaults along the Black Sea coast.

Mr Biden said his top priority at Thursday’s meetings was to make certain that the West stayed on the same page in its response to Russian aggression against Ukraine.

“The single most important thing is for us to stay unified,” he said.

Finland announced on Thursday it would send more military equipment to Ukraine, its second shipment in about three weeks. And Belgium announced it will add one billion euros to its defence budget in response to Russia’s invasion.

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President Joe Biden talks with European Council President Charles Michel (AP Photo/Evan Vucci)

At the same time, Washington will expand its sanctions on Russia, targeting members of the country’s parliament along with defence contractors.

The US said it will also work with other western nations to ensure gold reserves held by Russia’s central bank are subject to existing sanctions.

With Russia facing increasing international isolation, Nato Secretary-General Jens Stoltenberg also warned China against coming to Moscow’s rescue. He called on Beijing “to join the rest of the world and clearly condemn the brutal war against Ukraine and not support Russia”.

But Mr Stoltenberg, too, made clear that the West had a “responsibility to prevent this conflict from becoming a full-fledged war in Europe”.

The possibility that Russia will use chemical or even nuclear weapons has been a grim topic of conversation in Brussels.

Mr Stoltenberg said that Nato leaders agreed on Thursday to send equipment to Ukraine to help protect it against a chemical weapons attack.

White House officials said that both the US and Nato have been working on contingency planning should Russia deploy nonconventional weaponry. Nato has specially trained and equipped forces if there should be such an attack against a member nation’s population, territory or forces. Ukraine is not a member.

Mr Stoltenberg said in an NBC News interview that if Russia deployed chemical weapons, that would make “an unpredictable, dangerous situation even more dangerous and even more unpredictable”. He declined to comment about how the alliance might respond.