Moscow entered the Syrian civil war in 2015 on the side of President Bashar al-Assad's regime, and the country has been mired in a conflict marked by urban combat for more than a decade.

Now, US Department of Defense officials said, Russia's President Vladimir Putin was "on a recruiting mission" seeking to bring some of those fighters into the fray in Ukraine.

According to the Wall Street Journal, US officials said that Russia -- which launched an invasion into its Eastern European neighbor on February 24 -- has in recent days recruited fighters from Syria hoping they can help take Kyiv.

One official told the daily that some fighters are already in Russia readying to join the fight in Ukraine, though it was not immediately clear how many combatants have been recruited.

Details were slight: officials would not speculate on how many mercenaries have joined the fight, or on the quality of the fighters. But the Pentagon said there was no reason to doubt the accuracy of the reports.

"We do believe that the accounts of them -- the Russians -- seeking Syrian fighters to augment their forces in Ukraine, we believe there's truth to that," Pentagon spokesman John Kirby told journalists when asked about the reports.

But with enormous firepower and more than 150,000 deployed troops at 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," Kirby said, though he acknowledged the Pentagon does not have "perfect visibility" on exactly who was joining the cause.

Earlier Monday, a senior defense official told reporters more directly: "We know that they're trying to recruit Syrians for the fight."

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 traveled 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 centers across the country.

Russia's nearly two-week-old assault has seen more than 1.5 million people flee the country in what the UN has called Europe's fastest-growing refugee crisis since World War II.