A Russian call for fighters from Syria and Chechnya to join its war in Ukraine is motivated by plans to take major cities with urban warfare tactics which will require overwhelmingly superior attacking numbers, Western observers say.

After three weeks of fighting, all of the 150,000 Russian soldiers initially deployed at the Ukrainian border are now inside the country, experts say.

"The Russians have no more reserves in the zone," a source at the French military chiefs of staff told AFP.

But if President Vladimir Putin wants to capture major cities such as Kyiv and the Black Sea port of Odessa, he will have to bolster troops on the ground after an initial campaign phase that has been slower than the Kremlin expected.

This could equate to a numerical superiority of roughly 10 attackers for every Ukrainian soldier defending the urban centres, according to a military source, who asked not to be named.

Local knowledge, mobility and early occupation of vantage points typically favour defending forces in urban combat, adding the source.

The Kremlin said Friday that volunteers including from Syria were welcome to fight alongside Russia's military in Ukraine.

On Tuesday, a war monitor said Russia had drawn up lists of 40,000 fighters from Syrian army and allied militia ranks to be put on standby for deployment in Ukraine.

In a country where soldiers earn between $15 and $35 per month, Russia has promised them a salary of $1,100 to fight in Ukraine, the Observatory reported.

"Putin needs more troops than he thought he would. And he needs irregular troops because this war is becoming insurrectional," a Western security source said.

After failing to conquer Ukraine quickly, "Russia now needs massive reinforcements in terms of equipment and troops" to continue the war, said Mathieu Boulegue, a research fellow specialising in Russia at Chatham House, a think tank.

Moscow's recourse to Syrian soldiers follows on from Russia's intervention in 2015 in the war-torn country on behalf of the government, helping President Bashar al-Assad clock up decisive victories in the decade-long conflict.

Ramzan Kadyrov, strongman leader of Chechnya, claimed on Telegram Monday that he had been in Ukraine alongside Chechen fighters
Ramzan Kadyrov, strongman leader of Chechnya, claims he had been in Ukraine alongside Chechen fighters. SPUTNIK via AFP / Alexey NIKOLSKY

Russia's airforce notably helped Syrian forces during their siege of rebel-held Aleppo.

Syrian fighters have been deployed in foreign theatres before, by both Russia and Turkey in Libya, and by Turkey in Nagorno-Karabakh to help Ankara's ally Azerbaijan.

Putin has ordered his forces "to hold back on any immediate assault on large cities," Kremlin spokesman Dmitry Peskov said Monday. But, he added, the defence ministry "does not rule out" the possibility of putting large cities "under its full control".

Ukraine's President Volodymyr Zelensky slammed the Russian recruitment in Syria, saying Russia was hiring "murderers".

Fighters from Russia's overwhelmingly Muslim southern region of Chechnya are also loyal to the Kremlin and battle-hardened from reported past deployments in Syria and eastern Ukraine.

Numerous videos posted on social media have suggested that they are already present inside Ukraine.

Ramzan Kadyrov, the strongman leader of Chechnya, claimed on his Telegram channel Monday that he had been in Ukraine just outside Kyiv alongside the forces.

"You don't have much time left," he wrote in a taunting message to the Ukrainian leadership. "Better to surrender and join us," he said, adding in a gloating aside: "guess how close we are" to Kyiv.

Kadyrov, a devout Muslim who rules the Russian region in the northern Caucasus with an iron fist, is a former rebel turned Kremlin ally with a paramilitary force at his command.

At the start of the Russian offensive, images circulated on social networks showing a square in the Chechen capital Grozny filled with soldiers claiming to be on their way to Ukraine.

Kadyrov's forces are accused by rights activists of numerous abuses in Chechnya, including killings and enforced disappearances.

On Tuesday Kadyrov indicated he was back in Chechnya, welcoming to the region the secretary of Russia's national security council Nikolai Patrushev, a key member of Putin's inner circle.