A human rights group says Russia has sent about 2,000 mercenaries from Syria to Ukraine to bolster the invasion.

The Syrian Observatory for Human Rights says the mercenaries are believed to be conducting logistical support and other missions, primarily in Russian-occupied areas in eastern and southern Ukraine.

The group says that as of last month, roughly 50,000 people in Syria had signed up to fight for Russia. Such registration has been taking place in areas under the control of Syrian President Bashar al-Assad's Russian-backed government.

Recruiters in the northern city of Aleppo are purportedly telling people that they can earn 3,000 dollars a month and possibly acquire Russian citizenship if they enlist.

One Syrian man became a mercenary to provide for his wife and three children. He was sent to the eastern Ukrainian region of Donetsk in July.

The man later called his friend in Syria to dissuade him from traveling to Ukraine. He said Ukrainian military attacks had caused many deaths.

The man said Syrian mercenaries were treated like slaves and allowed only two meals of canned food per day. He added the food tasted rotten and that he was not sure what it had been made from.

The man said explanations given by recruiters were all lies. He said he had not been paid at all since he got 500 dollars in Syria.

The founder and director of the Syrian Observatory for Human Rights, Rami Abdulrahman, said Russian forces can push their frontlines and focus on battles against Ukrainian troops if Syrian mercenaries engage in logistical support.

He said that more Syrian mercenaries could be dispatched to Ukraine, depending on how the invasion develops.