The Russian government has dispatched its ''most ruthless'' special forces to war-torn Syria to track down and root out fleeing Islamic State (ISIS) militants and other armed enemies of President Bashar al-Assad, its lone Middle East ally, reports said.

Russia's feared Spetsnaz unit—the elite of the elite in Russia's army whose members have to undergo a gruelling training programme to earn their stripes—plus a covert paratroop battalion have arrived in Syria and are preparing for an all-out assault on ISIS militants and rebels fighting the Assad government, including moderate units such as the western-backed Free Syrian Army, according to the Daily Express.

"[President Vladimir] Putin's marines are there to guard the airbases they are using against sabotage by rebels,'' a military source said.

"But Spetsnaz and air-assault troops are not there to provide security to static objects. They are extremely aggressive and highly trained. They are there to mop up after air strikes, call in air strikes, go on extremely covert missions against rebels and ultimately wipe them out,'' the source added.

Spetsnaz are considered among the world's most efficient killers. The fearsome group together with the 7th Air Assault Mountain Division fought dirty wars in Chenchnya where they built up a deep hatred of jihadists, said the Express.

They reportedly went on bloody revenge missions against local jihad units to punish for atrocities against Russian troops and civilians.

A source told The Daily Mirror that the units will ensure Russia consolidates its position in the Middle East and will not be as accountable as British or U.S. special forces.

Moscow's latest move comes after Putin announced plans to send 150,000 troops to Syria to wipe out the terror group.

Since Wednesday, Russian warplanes have been flying over Syrian territory conducting air strikes on what Moscow says are targets belonging to ISIS jihadists in the country's northern and central provinces.

More than 50 facilities including a key command centre and a suicide belt factory have been destroyed as a result of Russia's merciless bombing campaign against the jihadis in recent weeks, The Daily Mirror reported.

However, NATO the other day called on Russia to cease strikes in Syria. It also warned against violating Turkey's airspace, saying in a statement that the allies "note the extreme danger of such irresponsible behavior."

Putin is claiming that the mission is to defeat the ISIS although the West maintains that Moscow's campaign is aimed more in helping prop up the Assad regime.

The United States has also accused Russia of using the raids as cover to hit moderate opponents of Russia's ally, Assad.