Ukrainian President Volodymyr Zelenskyy wants to build an "International Legion" of foreign volunteers to fight Russia. Germans are certainly among those that Russia derides as "mercenaries" – but no one really knows how many there are.

The war caught up with Siegfried one day before his planned departure from Ukraine.

Detonations woke him up, he said by telephone from Kyiv some two weeks later. "It was clear then that the war had begun," he said.

The German, who did not want to give his full name, stayed and joined the Ukrainian volunteer army, the so-called territorial defense. As for his motive, he said, "I thought it was too cowardly and too stupid to run away from this fight."

Now, in his early 40s, he is a platoon leader, he said. Under his command are about 30 people in a unit preparing to defend the Ukrainian capital as reinforcements for the armed forces.

All they have are simple weapons like Kalashnikov assault rifles, but they desperately need protective vests, vehicles and night-vision equipment – and more weapons in general, he urged.

As a young man, he did his military service with the German paratroopers. He is also a trained technician and worked as a trucker in Canada, Siegfried explained. From 2015 onward – after the annexation of Crimea and because of the conflict in the Donbass region – he joined Ukrainian forces for several years.

Now he has joined in a different capacity. "The only answer that will stop the Russian army from carrying out further attacks is a military defeat," he said.

According to Ukrainian government circles, several hundred German citizens have traveled to Ukraine to fight the Russian army.

Ukrainian media is abuzz with estimates on how many foreigners in total have signed up for a so-called "International Legion."

Some quote figures of more than 20,000 volunteers from 52 countries, although these cannot be verified.

Deputy Interior Minister Yevheniy Yenin did not want to give any concrete figures on Ukrainian television this week, saying only that "their number is increasing."

The border and migration authorities arrange entry into Ukraine, and then the volunteers are handed over to the Defense Ministry.

"They receive a corresponding document and receive a military ID card, which replaces the residence permit," Yenin explained.

In the future, they will be able to apply for Ukrainian citizenship.

They are not, however, mercenaries, the Ukrainian armed forces are at pains to explain.

"They are volunteers, there is no pay," a government source said. "They should not be accused of fighting for money," it added.

Without border controls with neighboring countries such as Poland, the German authorities say they cannot estimate the numbers either, and only have known right-wing extremists on their radar. Three such individuals have traveled to Ukraine, according to Deutsche Presse-Agentur (dpa) sources.

Ukrainian President Volodymyr Zelenskyy has made a great deal of the International Legion.

"They are coming to defend freedom, life. For us, for all of us," he announced. Help is being solicited on the internet and social media aimed at "all the heroes of the free world."

Russia has not shied away from threatening these foreign fighters.

"All mercenaries sent by the West to help the nationalist Kyiv regime ... have no right to the status of prisoners of war," Defense Ministry spokesperson Igor Konashenkov said, according to the Interfax news agency.

Russia will "hold them criminally responsible if they are apprehended."

From Germany's point of view, Ukrainian citizens living in Germany can travel to their home country without penalty in order to participate in the conflict there. This also applies to dual German-Ukrainian nationals, although they could in theory lose their German citizenship.

The case is different if volunteers have German citizenship only. If someone from this group takes part in combat operations, international humanitarian law is the key reference point for possible criminal liability, a spokesperson for the Justice Ministry explained.

"If an act of killing or injury is permitted under international law, then it is not punishable under German criminal law," the spokesperson said.

Nevertheless, you won't find anyone openly talking about recruitment in Berlin.

Section 109h of the German Penal Code states that "whoever recruits a German for the benefit of a foreign power for military service in a military or military-like institution or supplies him to its recruiters or to the military service of such an institution shall be punished with imprisonment from three months to five years."

Even attempting to do so is a crime.