The Ukrainian embassy in Washington D.C. has received more than 3000 applications from US citizens who want to join the fight against Russia.
The official told Military Times that hundreds of volunteers had arrived in Ukraine. He could not say how many veterans the applicants include.
On Sunday, Ukrainian President Volodymyr Zelensky announced the formation of an International Legion for foreigners who want to fight against Russia.
The statement on the presidential website said that anyone who wants to join the defense of Ukraine, Europe and the world can come and fight side by side with the Ukrainians against the Russian war criminals.
Liz Truss, the UK's foreign secretary, said on Sunday that she supported British people who want to fight in Ukraine.
She said that the people of Ukraine were fighting for freedom and democracy for the whole of Europe.
Ukrainian officials told Military Times that they are looking for volunteers with combat experience who would require little training. To join the newly formed International Legion, volunteers must have proof of military service or work with law enforcement agencies.
We don't want to send people who aren't trained, but at the same time, people can provide medical help. The embassy official told the Military Times that some can analyze mass media if they don't want to fight.
It is legal for Americans to join the International Legion if they have served in the armed forces of a foreign country.
It could be a violation of the law if someone has been recruited or hired in the United States. Ukrainian officials say they are not hiring anyone and are only opening the door for volunteers to help.