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Russia is recruiting Syrians and other foreign fighters to reinforce its assault on Ukraine, the Pentagon has said.
President Vladimir Putin was seeking to bring some of the fighters involved in the Syrian civil war into the fray in Ukraine, the US Department of Defence officials said on Monday.
Moscow entered the war Syria war in 2015 on the side of President Bashar Al Assad's regime.
Meanwhile, Mr Putin said he will not send conscripts or reservists to fight in Ukraine.
“Conscripted soldiers are not participating and will not participate in the fighting,” Mr Putin said in a televised address. “There will not be an additional conscription of reservists either.”
Russia, which attacked its Eastern European neighbour on February 24, has in recent days recruited fighters from Syria, hoping they can help take the Ukrainian capital of Kyiv and other cities, US officials told the Wall Street Journal.
Some fighters are already in Russia preparing to join the fight in Ukraine, although it was not immediately clear how many combatants have been recruited.
But even with enormous firepower and more than 150,000 deployed troops at Mr Putin's disposal, the Pentagon said it was noteworthy that he would find it necessary to recruit mercenaries.
“It's interesting that Mr Putin would have to find himself relying on foreign fighters here,” Pentagon spokesman John Kirby said, acknowledging that the Pentagon does not have “perfect visibility” on exactly who was joining the cause.
Foreign combatants have already entered the Ukrainian conflict on both sides.
Chechnya strongman leader Ramzan Kadyrov, a former rebel-turned-Kremlin-ally, has shared videos of Chechen fighters joining the attack on Ukraine and said some had been killed in the fighting.
Ukrainian Foreign Minister Dmytro Kuleba has claimed around 20,000 foreign volunteers have travelled to the country to join Kyiv's forces.
The capital and the second-largest city, Kharkiv, are still held by Ukraine's government, while Russia has seized the port city of Kherson and stepped up its shelling of urban centres across the country.
Russia's nearly two-week-old assault has led to more than 1.7 million people fleeing the country.