Russian President Vladimir Putin said Russian private military contractors from a company known as the Wagner Group had the right to work and pursue their interests anywhere in the world as long as they did not break Russian law.
The Kremlin has until now denied suggestions that Russian contractors are operating clandestinely in Syria.
"I repeat, they are not breaking Russian law and have the right to work and promote their business interest wherever they like in the world," Putin told his annual news conference.
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"If this Wagner Group is breaking any laws, the prosecutor's office must give its legal opinion."
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Putin said he agreed with Trump's decision to pull 2,000 U.S. troops out of northern Syria.
"I don't think they're needed. Let's not forget that the presence of (U.S.) troops there is illegitimate. The U.S. is there without backing from the United Nations or an invitation from the Syrian government. Russia is there at the invitation of the Syrian government. But if the U.S. has decided to withdraw, that's good," he said.