The exact number varies, ranging between 10,000 and 20,000 according to their estimates. However, it is unclear exactly who came from where.
Some of the volunteers have come from Libya and Syria, the latter of which is ruled by Russia-aligned President Bashar Assad. Others likely came from the Wagner Group, a Russian paramilitary company heavily involved in Russian military operations around the world and who have been reported to be extensively active in the ongoing conflict.
These foreign fighters, however, are not coming with more equipment or tools.
"They don't have any heavy vehicles and weapons. It's much more infantry," the official claimed, according to Western media reports. "These guys are mainly used as a mass against Ukrainian resistance."
US defense officials were unable to comment on the veracity of this exact figure. However, they have confirmed that Russia is indeed looking to recruit foreign fighters and that Wagner mercenaries have been working to transfer manpower from North Africa.
In addition, an anonymous senior US defense official said that Russia added two more combat units, known as battalion tactical units. This brings the current number of these units in the south and east of Ukraine to 78. As the Pentagon had said at the beginning of the invasion that the typical unit had 700-800 soldiers, 78 units would mean 55,000-62,000 soldiers, according to The Associated Press. However, the exact number of soldiers at this time is unclear.
The greenlit 16,000
Back in mid-March, Russian President Vladimir Putin approved to Defense Minister Sergei Shoigu the plan to allow some 16,000 Syrian volunteers to fight alongside Russian forces in Ukraine.
"We know many of [these volunteers], they helped in the fight against ISIS during the last 10 years," Shoigu had said at the time.
A few days later, the GUR reported that Russia was training Libyans and Serbians to fight in Ukraine.
These 16,000 troops would fit among the 10,000-20,000 foreign fighters this European official claims are fighting in Ukraine alongside Russia.
But when asked, the US Defense Department was unsure.
"The Russians themselves said they were going to recruit 16,000 Syrians," Pentagon press secretary John Kirby told reporters in a Tuesday press briefing.
"We don't know whether they actually achieved that goal, or how close they got to it and how many foreign fighters there might be in Ukraine, we just don't have a good count."
Michael Starr and Jerusalem Post Staff contributed to this report