Pentagon spokesman John Kirby confirmed on Monday that Russia is recruiting Syrians and other foreign fighters as it ramps up its assault on Ukraine, AFP reports.

The comments by Kirby followed a report in the Wall Street Journal on Sunday which cited US officials who said that Moscow is recruiting Syrians skilled in urban combat to fight in Ukraine as Russia’s invasion is poised to expand deeper into cities.

An American assessment indicates that Russia, which has been operating inside Syria since 2015, has in recent days been recruiting fighters from there, hoping their expertise in urban combat can help take Kyiv and deal a devastating blow to the Ukraine government, said four American officials quoted in the report.

"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," Kirby told journalists on Monday when asked about the reports.

"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."

Moscow entered the Syrian civil war in late 2015, providing, among other things, air support for the army of Syrian President Bashar Al-Assad.

While Russia claimed its air strikes were targeting only the jihadist Islamic State (ISIS) group, Western officials said the strikes also targeted Western-backed "moderate" rebel groups which Assad views as “terrorists”.

Russia also provided Syria with the advanced S-300 anti-aircraft system, which fires missiles from trucks and is designed to shoot down military aircraft and short and medium-range ballistic missiles.