Moscow noted the transfer of 540 military contractors in June alone and says the mercenaries are tasked with protecting oil facilities and training militants loyal to the US.
Russia says the US is replacing regular army troops with private military contractors in northern Syria, putting the total number of mercenaries in the thousands.
Russian Foreign Ministry spokeswoman Maria Zakharova told a news conference that in June alone 540 people including 70 including instructors and commanders entered the country.
"The number of personnel of PMCs [private military companies] in Syria exceeds 4,000,” she said. “The transfer of mercenaries is carried out by car in groups of 12-16 people.”
Mercenaries were brought in to train militants loyal to the US and protect oil facilities, according to Zakharova.
The militants in question belong to the local Syrian branch of the PKK terrorist group, known as the YPG.
The PKK, which is proscribed as a terrorist organisation by the US and other Western states, is responsible for the killing of tens of thousands of civilians and security forces in Turkey.
They are allied to the US under the banner of the Syrian Democratic Forces (SDF) and control vast swathes of Syria with air protection provided by Washington.
Identity of mercenaries unclear
The identity of the mercenaries is not clear from Zakharova’s comments but groups, such as XE, which is headed by former US Navy Seal Eric Prince, have worked alongside the US in war-affected countries, such as Iraq and Afghanistan.
Mercenaries contracted, formerly known as Blackwater, have been accused and convicted of war crimes targeting civilians.
In December 2018, a former Blackwater mercenary, Nicholas Slatten, was convicted of murder by a US court for his role in the massacre of dozens of Iraqis in the 2007 massacre at Baghdad’s Nisour square.
Syria is in the ninth year of a civil war, which started in 2011 after peaceful protests against the regime of Bashar al Assad were brutally put down by his forces.
Since then, more than 11 million people have been displaced by fighting, as well as the atrocities carried out by the Assad regime. Around six million have fled abroad, while roughly five million are internally displaced.
According to the United Nations, more than 500,000 people have been killed in the conflict.