In recent weeks, many of the thousands of foreign volunteers who have flocked to join the fight against Russia have gone missing or been captured.

Last week, two Britons and a Moroccan who were taken prisoner while fighting for the Ukrainian armed forces were sentenced to death in Russian-occupied eastern Ukraine after being charged with terrorism.

This week, two Americans fighting with a group of foreign soldiers went missing in action near Kharkiv, Ukraine’s second-largest city, about 40 kilometers from the Russian border. Their families fear they have been captured, having disappeared after the platoon came under fire.

The missing and captured fighters have drawn attention to the thousands of largely unregulated volunteers in Ukraine, only some of whom have been accepted into the Ukrainian army’s International Legion.

The platoon to which the missing Americans belonged was one of dozens of loosely organized volunteer groups that absorbed foreign veterans, many of them Americans. The volunteers have proven both valuable assets and sometimes an unruly problem for Ukraine, and present a potentially difficult challenge to their home governments if captured or captured.

On Friday, President Biden said he had been notified of the two Americans missing in Ukraine and the administration did not know their current locations.

“I want to reiterate: Americans should not go to Ukraine now,” he said.

The International Legion, formed after Ukrainian President Volodymyr Zelensky appealed in late February for foreigners to help fight, is considered the most selective of foreign groups.

Damien Magrou, a Franco-Norwegian lawyer who is the spokesman for the Ukrainian army’s International Legion, said in an April interview that he felt the war had “stripped a chord” among many American veterans. .

“There are also a lot of American veterans who think they can make a difference because the United States has been involved in far more conflicts over the past 20 years than European countries,” he said. .

Mr Magrou, a corporal in the structure of the legion, said volunteers accepted must now have combat experience, no record of disreputable behavior and no membership in extremist groups. Other groups aren’t as selective, he said.

Mr Magrou said he encouraged volunteers rejected by the legion to take a shuttle provided by the army back to the Polish border. But, he added, “they are in the country legally and we cannot force them to do anything.”

Russia maintains that some of the foreign fighters it has captured are mercenaries and are not entitled to protection as prisoners of war under international law. A local court in the Russian-occupied region of Donetsk found that the two British fighters and a Moroccan, who had immigrated to Ukraine, were guilty of “training for the purpose of carrying out terrorist activities” and that they had undertaken their activities “for remuneration”. .”

It remains unclear what missions were carried out by the group whose American members disappeared, or who, in the Ukrainian armed forces or the government, supervised them and gave them orders.

The missing US veterans are Alex Drueke, 39, a former US Army staff sergeant who served in Iraq, and Andy Tai Ngoc Huynh, 27, a former Marine, family members said. They disappeared when their platoon came under “heavy fire” in a village on June 9, causing all but the two of them to withdraw, according to a statement from Mr. Drueke’s family. Reconnaissance on foot and by drone revealed no signs of the two soldiers, the statement said.

The Geneva Conventions, which govern the law of war and which Russia has signed, specify that captured volunteer combatants may also be considered prisoners of war. The main definition of a mercenary under international law is someone who fights primarily for financial gain and is paid significantly more than the local armed forces.

Those who join the International Legion receive the same amount as their Ukrainian military counterparts. They receive a base salary, equivalent to approximately $630 per month, with bonuses of up to several thousand dollars per month.

Some fighters with other groups receive one-time payments to cover their expenses, while others are unpaid.

Lawrence Hill-Cawthorne, an associate professor of law at the University of Bristol, said even volunteer fighters not integrated into the Ukrainian army would be entitled to POW protection if they openly carried arms while fighting.