Russia is recruiting Syrian fighters experienced in urban combat as it ramps up its assault on Ukraine, according to US officials quoted by the Wall Street Journal on Sunday.

Moscow, 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, four US officials told the US daily.

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

One official told the Journal 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, and the sources would not provide further detail.

Foreign fighters 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 assault, now in its twelfth day, 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.