Ukraine has set up a new unit, encouraging foreign volunteers to join its fight against invading Russian forces.
President Volodymyr Zelensky, who has stayed in the capital Kyiv, announced the formation of a new 'international legion' to help defend his country.
Dmytro Kuleba, Ukraine's Foreign Affairs Minsiter, wrote on social media: "Foreigners willing to defend Ukraine and world order as part of the International Legion of Territorial Defense of Ukraine, I invite you to contact foreign diplomatic missions of Ukraine in your respective countries. Together we defeated Hitler, and we will defeat Putin, too."
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Foreign Secretary Liz Truss has said she would "absolutely" support British nationals who choose to go to Ukraine to help fight the Russian invasion.
"That is something people can make their own decisions about," she told BBC's Sunday Morning programme.
"The people of Ukraine are fighting for freedom and democracy, not just for Ukraine but for the whole of Europe.
"Absolutely, if people want to support that struggle I would support them in doing that."
There have in the past been questions about the legality of Britons going abroad to fight in foreign conflicts.
In 2014, the Crown Prosecution Service warned that UK nationals who went to fight in the Syrian civil war could be committing an offence, even if they joined the rebels fighting to overthrow President Bashar Assad.
Mr Zelensky has banned men of military age, 18 to 60, from leaving the country.
While hundreds of thousands of refugees are leaving Ukraine amid Russia's attack on the country, some Ukrainian men and women are returning home from across Europe to help defend their homeland.
Poland's Border Guard said on Sunday that some 22,000 people have crossed into Ukraine since Thursday when Russia invaded the country.