Britain’s top diplomat says she would support people going to fight for Ukraine.

Britain’s foreign secretary, Liz Truss, said on Sunday that she would support Britons who want to go to Ukraine to take up arms to resist the Russian invasion, in an intervention likely to escalate tensions with Moscow.

When asked in a BBC interview about a call from President Volodymyr Zelensky of Ukraine for foreigners to join his country’s fight against Russia, Ms. Truss said those resisting the Russians were “fighting for freedom and democracy not just for Ukraine but for the whole of Europe.”

Asked specifically whether she would support Britons going to Ukraine to fight, she responded: “Absolutely, if that is what they want do to.”

Like other NATO nations, Britain has ruled out deploying its own troops to Ukraine, but Ms. Truss said the government was doing all it could to support Ukraine by supplying it with defensive weapons.

Earlier, on the same BBC broadcast, Ukraine’s ambassador to Britain, Vadym Prystaiko, said that there had been an “overwhelming number of people” from many nations coming to his embassy, offering to fight.

“We don’t want to break any laws, especially international laws, but most of these people who are coming, they are Ukrainians. They have the right to come home at any moment and fight,” he said.

There is legal uncertainty over the matter because Britain’s Foreign Enlistment Act of 1870 bars people from joining an army in conflict with any foreign state that is “at peace” with Britain, although this law was not used against Britons who fought against Spain’s fascist government during the Spanish Civil War of the 1930s. More recently, British citizens have joined foreign military forces, including the French Foreign Legion or the Israeli Defense Forces.

By contrast, some of those who fought for Islamic State in Syria have been stripped of their British citizenship to prevent their return.