U.S. Marines are barred from traveling to Ukraine and two neighboring countries, while those going on official or personal travel in Europe must now request prior clearance for their trips, the service said Thursday.

“Due to the ongoing armed conflict in Ukraine, Marines are prohibited from travel to Ukraine, Belarus, and Moldova,” the Marine Corps said.

The directive said Marines and other military personnel attached to Marine units who are going anywhere in Europe on vacation or temporary duty must first submit travel clearance requests to the Aircraft and Personnel Clearance System, effective immediately.

The purpose of the message, known as a MARADMIN, was to echo a State Department “Do Not Travel” advisory and travel restrictions on all service members that the head of U.S. European Command imposed in February, said Maj. Jim Stenger, a Marine Corps spokesperson.

“I know I check MARADMINs more frequently than State Department travel advisories, so this was an attempt to communicate an important message to our Marines and sailors directly,” Stenger said.

It comes one month into the Russian invasion of Ukraine and follows reports of thousands of foreigners responding to Ukrainian President Volodymyr Zelenskyy’s call for volunteers to help repel the advance.

But some U.S. military veterans have been frustrated after arriving and being assigned to comparatively menial infantry tasks relative to their military specialties, Military.com reported this week.

Meanwhile, Russia has warned that foreigners will not be treated as legitimate combatants if captured.

Also this week, a Canadian defense official told members of Parliament that the country’s military members are now barred from joining a new international wing of Ukraine’s Territorial Defense Forces, the Guardian reported.

Enough volunteers from America’s northern neighbor had answered Zelenskyy’s call that one source told the National Post early this month that they had formed their own unit, the Canadian Ukrainian Brigade.

But Lt. Gen. Frances Allen, vice chief of defense staff, told members of Parliament on Wednesday that officials were concerned Moscow would use captured Canadian troops in disinformation campaigns. 

Stenger, the Marine Corps spokesperson, declined to say why that service mandated travel clearances for trips throughout Europe, or whether it stemmed from concerns that Marines might try to sneak their way into battle as freelancers.

“I appreciate your questions but we have nothing further to add on this,” Stenger said in an email.

Daniel Carde, a photojournalist in Ukraine, wrote on Twitter that he met a Marine reservist from Utah who’d traveled to the western city of Lviv to take part in the fight.

“He said his unit doesn’t know he’s here,” Carde said.